Modify state storage to handle multiple members in the same group

[mulmarta]

In general, there is no reason why a storage can't support multiple members in a single group. The new group state identifier is therefore (group_id, leaf_index) instead of just group_id.

Group states stored in the old way can be recovered using load_group(group_id).

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT license.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 99.21875% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 89.38%. Comparing base (5235fc8) to head (fd7d8b2).

Files	Patch %	Lines
mls-rs/src/group/state_repo.rs	98.41%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #124 +/- ## ========================================== + Coverage 89.34% 89.38% +0.03% ========================================== Files 173 173 Lines 31375 31454 +79 ========================================== + Hits 28032 28114 +82 + Misses 3343 3340 -3 ```

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