Add SuccinctRoles class containing the information from the
succint_roles dict described in TAP 15.
This allows for easy mypy checks on the types, easy enforcement on
TAP 15 restrictions (as for example that "bit_length" must be between 1
and 32) and support for unrecognized fields inside succinct_roles
without much of a hassle.

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

Add zero padding to bin names inside SuccinctRoles.
Zero padding ensures that the bin names always have the same length.

This characteristic is implied in the example given by TAP 15 where
the third bin is named "alice.hbd-03". For context read TAP 15:
https://github.com/theupdateframework/taps/blob/master/tap15.md

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

Add two helper methods in SuccinctRoles.
Those methods proved useful in the testing code, but I believe they have
a potential value for production code as well.

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

Clarify explicitly that exactly one of "paths" and "path_hash_prefixes"
must be set inside DelegatedRole.
Also simplify the check for "paths" and "path_hash_prefixes".
Finally, add a test case inside the "test_metadata_serialization.py"
test file about wrong keyids type for "Role" serialization.

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

This commit contains 2 API changes in "Delegations" class from
tuf/api/metadata.py:
1. roles argment is made optional
2. unrecognized_fields argument becomes the 4-th rather than the 3-rd
as it used to be

In this commit, I add support for succinct_roles roles inside
Delegations class. This change is related to TAP 15 proposal.

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

Here is the list of all breaking API changes:
1) The "role" and "key" arguments in "Root.add_key()" are in reverse
order - "key" becomes first and "role" second.
2) "Root.remove_key()" has been renamed to "Root.revoke_key()".
3) The "role" and "keyid" arguments in "Root.revoke_key()" are in
reverse order - "keyid" becomes first and "role" second.
4) The "role" and "key" arguments in "Targets.add_key()" are in reverse
order - "key" becomes first and "role" second.
5) "Targets.remove_key()" has been renamed to "Targets.revoke_key()".
6) The "role" and "keyid" arguments in "Targets.revoke_key()" are in
reverse order - "keyid" becomes first and "role" second.
7) In both methods "Targets.add_key()" and "Targets.revoke_key()" the
"role" argument becomes an optional with a default value of None.

Those changes are made in an effort to make those methods logical
for both cases when standard roles and succinct_roles are used.
The "Root" API change was done in order to preserve naming and argument
order consistency with "Targets" API.

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

Add support for Targets using delegation with succinct_roles.
For that purpose, we needed a method that can add succinct_roles
information with its all corresponding bins to the target metadata
and self.md_delegates attribute in RepositorySimulator.

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

Test traversing the delegation tree when there is a Targets using a
delegation with succinct roles.

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>