[uniffi] Remove `GroupState` type from storage trait

[mgeisler]

Issues:

Addresses #81

Description of changes:

This removes the GroupState type from the GroupStateStorage trait in mls-rs-uniffi. This is done for two reasons:

• It makes all methods on the trait consistent in the way that they take a group_id as the first argument.

• Fewer types means less boiler-plate generated by the UniFFI bindings generator (Vec<u8> is a built-in type in UniFFI).

Call-outs:

The EpochRecord type stays: I don’t think we have a different way of passing in a vector of epochs IDs with their data.

I could apply this change to mls-rs too, but it doesn’t seem useful there since it’s cheap and easy to create a new type in Rust.

Testing:

Updated tests.

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

[mgeisler]

I don't think this is super important for us, it was just something that came up when Anh was looking at the storage trait.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.49%. Comparing base (e4009f9) to head (ebb0493).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #128 +/- ## ========================================== - Coverage 89.49% 89.49% -0.01% ========================================== Files 173 173 Lines 31407 31404 -3 ========================================== - Hits 28109 28106 -3 Misses 3298 3298 ```

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