Add config type for joining from an external commit

[keks]

This PR adds a new sort of join config to make joining using external commits easier. I believe it's mostly pretty obvious, so just a few things:

• The types have a generic lifetime parameter, because aad has been a reference in the past and I wanted to avoid needless copying.
• I am not quite sure about which of the arguments belong into the config and which should be arguments. The ones I included are more on the "small data" side, whereas things like group info and ratchet tree are more on the "large protocol objects" side. I wasn't sure what to do with the credential_with_key. Feedback on this is very welcome.

Fixes #1607

[codecov]

Codecov Report

Attention: Patch coverage is 72.56637% with 31 lines in your changes missing coverage. Please review.

Project coverage is 84.85%. Comparing base (5fb5e62) to head (ce247cf).

Files	Patch %	Lines
openmls/src/group/mls_group/config.rs	65.55%	31 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1614      +/-   ##
==========================================
- Coverage   84.92%   84.85%   -0.08%     
==========================================
  Files         187      187              
  Lines       21692    21786      +94     
==========================================
+ Hits        18423    18486      +63     
- Misses       3269     3300      +31     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[raphaelrobert]

Thanks for tackling it! I think AAD really is the outlier here and I should get working on #1521 instead. I thought about it some more, and the current way the AAD is handled only came into existence because I didn't have any better ideas at the time, not because it's actually how folks are using it. All discussions I've had so far point to wanting an ad-hoc solution for the AAD. And since it's a breaking change for the storage provider, it would be good to get this in for v0.6. I'll get started with it.

[keks]

Just a note: this PR also removes the --message-format json from the cargo rustdoc check in CI so it's more readable.

[keks]

Do you want to do your aad changes inside this PR or do you want to do it separately @raphaelrobert? Not sure how to interpret your self-assign :)

[raphaelrobert]

Do you want to do your aad changes inside this PR or do you want to do it separately @raphaelrobert? Not sure how to interpret your self-assign :)

Self-assign is always for reviews. The PR is already is here already: #1615, but I'm still adding tests. I guess it would make sense to get in #1615 first because then you can drop the lifetime.

[franziskuskiefer]

What's the plan here? @keks did you want to close this?

[keks]

On a call we discussed restructuring this such that the external commit join config contains the regular join config, instead of mirroring all the fields. With that, and the changes to the AAD, I think it would make most sense to just start over on top of master, if we want to get this in.

Should I have another stab at this or should we close this unfinished?