Check for last resort in core_group/new_from_welcome

[josephlukefahr]

Closes #1502

This PR just copies the logic in group::MlsGroup::new_from_welcome(...), which checks for last resort extension in keypackage, to group::CoreGroup::new_from_welcome(...).

[kkohbrok]

Thanks for the PR! It looks like it solves the issue. To confirm that it does and to ensure we don't run into this sort of thing in the future, would you be so kind as to add a simple test? You can add the test to mls_group/test_mls_group.rs, where you can find inspiration on how to set up the test as well. I think it's better to have the test at the MlsGroup (rather than the CoreGroup) level, since we're likely to move all provider access to MlsGroup level eventually anyway (see #1207).

(Also you'll have to run cargo fmt on the code before we can merge.)

[josephlukefahr]

See 050d330 for the test. Open to feedback/guidance on robustness of the test procedure for this case.

[josephlukefahr]

See 4253a26 for the cargo fmt changes.

[kkohbrok]

Thanks for adding the test and making the requested changes! This looks good to me, but clippy has a few complaints. Once you sort those out we can get this merged.

[kkohbrok]

Sorry, one last thing: Could you add an entry to the changelog indicating that this bug was fixed? You can find it in CHANGELOG.md.

[kkohbrok]

Excellent, thanks!