Expose the KeyPackage held by AddProposal to crate consumers

[jszczepanik]

Description of feature:

mls_rs::group::proposal::AddProposal::key_package should be available to consumers of the crate.

Use case:

Some custom implementations of MlsRules could benefit from being able to inspect the KeyPackage when processing the AddProposal.

Implementation discussion (Optional)

It a matter of adding a getter method on AddProposal.