Update InboxV2 to use IdentityProvider

[jazzz]

This PR is the sibling PR to #102. It updates InboxV2 to use and abstract IdentityProvider, rather than requiring the internal LogosAccount implementation.

Changes

• Removes the direct dependency on LogosAccount within InboxV2 and GroupV1 in preparation for its removal.
• Remove MLSContext as suggested by @kaichaosun. Instead the functionality is flattened to mimic a OpenMLSProvider. This enables future implementation of MLS storage.

Deferred Work:

• The context interface is left unchanged. Once this and Add crate logos-accounts #102 land, the interface, examples and tests can be updated. This keeps the PR smaller.
• Removal of conversations::LogosAccount. The internal representation is still used. It can be removed once the Context constructors accept external IdentityProvider Impls.

Notes

• This Depends on Remove naming conflict with Signatures #101 so its currently marked as Draft

Future PRs

• Remove generics at in GroupV1.
• Remove Runtime borrow checks in GroupV1

[kaichaosun]

It should be identity, right?
Also I'm wondering if we can derive account_id from IdentityProvider?

[jazzz]

Hmm great point. We need to lock down our language for the Accounts/Identity Model. Will handle offline

[kaichaosun]

Should we inject the storage, so it can be easier to be replaced later with actual store engine.

[jazzz]

This is only ever initialized in a single location - Its east to change the types.

Skipping injection for now, as we can solve the problem later.

[kaichaosun]

Good to merge once clearness is finished.