Add more context to ms rules / identity provider callbacks

[mulmarta]

Maybe useful to know more context

Call-outs

Moving GroupContext to core means making tree_hash and confirmed_transcript_hash public (there's currently a function on ExternalGroup that returns confirmed_transcript_hash anyway).

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

[tomleavy]

@mulmarta holding off on merging this for a week or so to see if we need any non-breaking changes first

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.91304% with 14 lines in your changes missing coverage. Please review.

Project coverage is 90.16%. Comparing base (9f4d07f) to head (431ba42).

Files with missing lines	Patch %	Lines
mls-rs-core/src/group/context.rs	57.89%	8 Missing :warning:
mls-rs/src/group/mls_rules.rs	55.55%	4 Missing :warning:
mls-rs-identity-x509/src/provider.rs	0.00%	1 Missing :warning:
mls-rs/src/group/message_processor.rs	90.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #204 +/- ## ========================================== + Coverage 90.14% 90.16% +0.01% ========================================== Files 175 176 +1 Lines 31498 31558 +60 ========================================== + Hits 28395 28453 +58 - Misses 3103 3105 +2 ```

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[mulmarta]

I fixed the merge conflict (version in Cargo.toml), I think I addressed all the comments, and we waited 2 weeks. OK to merge @tomleavy ?

[tomleavy]

I fixed the merge conflict (version in Cargo.toml), I think I addressed all the comments, and we waited 2 weeks. OK to merge @tomleavy ?

Yup, I think we are good