Adds a group info config type, which tracks distributed info for v2
groups.

This requires introducing/using a few new concepts not currently used
for user config messages:

- Multiple decryption keys.  User config doesn't do this at all (rather
  it generates a single decryption key from the private key for each
  namespace).

This doesn't yet add support for sharing and distributing those keys,
just for having being able to load a config with a list of multiple
possible keys.

- Config signing and verification.  For user configs this isn't
  done, since only the owner can actually encrypt/decrypt a config
  message, just being able to decrypt it is authentication enough.

This required various modifications to make the config library properly
prevent modifications when we can't modify, and to properly follow
that through in terms of merging, updates, etc.

Fixes a bug where construction-provided keys were loaded in reversed
priority.

Tests to follow.

The first part of this assert shouldn't fail if we are starting from a
fresh, blank config (in which case we don't actually serialize our own
message anymore).

It doesn't really make sense to have this as we either want signatures
(for shared messages) or don't (for personally encrypted messages).  The
only place we were passing it as `true` was in a place that also didn't
pass a verifier or signer (during dumping), and so already wasn't
adding/checking signatures.

This makes Keys construction, loading, and rekeying take the Info and
Members object to update their keys, thus removing the requirement for
application code to worry about key management at all (they just feed
the keys in, and they propagate to info/members).

Also adds a state dump (similar to base config `dump()`) to Keys.

Messages to a new group can't go into 0 (since that is publicly
depositable), so reserve a namespace (11) for messages.

Adds method for encrypting/decrypting a message.

This supports both compressed+encrypted and just plain encrypted.

Abstracts the zstd compression implementation from base.cpp into
internal.cpp, and uses it inside the new group message encryption

Compression is only used if beneficial (that is, only if compression
actually reduces the message size).

- new group keys unit tests runs through relevant functionalities
- add/remove users, change group info, add admin, verify encryption compression

- add test
- add missing `erase_group` to erase by id
- fix bug where `K` wasn't set for non-admins (it should be set but
  empty in such a case so that we always have a key).
- make `check_session_id` take a string view prefix instead of a char to
  make it a little easier, and to include it in the error message.

It was temporary for simpler keys testing.

This adds methods to `Keys` that generates subaccount tokens and
signatures as needed to do storage server subaccount authentication
(which currently requires testnet as the subaccount code is not yet
active on mainnet), along with test code to test it.

Also adds a tests/swarm-auth-test binary that spits out storage requests
for store/retrieve testing.

It adds a nlohmann::json dependency, and isn't really part of the normal
test suite.

- Adds C wrappers for all the swarm authentication methods
- Adds C wrapper for querying if the keys object has admin permission
- Adds C wrapper for key_supplement to generate supplemental key messages

- Changes the swarm `subaccount` value to be base64 instead of hex
  (since it isn't really a pubkey)
- Makes the vector passed into key_supplement const&

- Add keys.size() return the number of keys in the object.
- Add C API for retrieving keys & key size
- Fix copy-and-paste error in groups_members_size C API name

Also adds tests to verify that auth_data and secretkey data propagate as
expected.

Many thanks to Harris for identifying and tracking this down.

- Added config_groups_keys to config/base.h
- Looped in test call into config user groups C api unit test

Adds current hash tracking to groups::Keys; this adds a `current_hashes`
method to group::Keys that works similarly to the base config
current_hashes (though returns a set instead of vector).

(The C version is `groups_keys_current_hashes`)

Testing this also triggered a bug in that we weren't probably re-loading
the verified-signature state on a dump->load cycle, which caused an
assertion failure on merge (because the current state couldn't be
successfully serialized-then-deserialized), also fixed here.

Also renames the recently added C config_group_keys to config_get_keys
to better reflect its purpose.

This adds an `.invited` flag for all group types (legacy, new, and
communities) that can be used to track a invited-but-not-yet-joined room
by session clients.

Also adds `.name` to new groups data so that the name from an invitation
can be stored before accepting (after accepting the name will come from
the shared group config message).

Add `setKicked()` and `kicked()` methods which clear both auth_data and
secretkey and report whether both are empty, respectively.  (For the C
API these are `ugroups_group_set_kicked` and `ugroups_group_is_kicked`
and take a pointer to the `ugroups_group_info`).

The key lists weren't being set properly when loading a Keys object from
a dump; these sets the key lists properly.