Mutable Metadata group name Part 2

[cameronvoell]

We would like to have a metadata extension type that can be modified after group creation. This is possible using the Unknown Group Context Extension type, which accepts a byte array as the only field. See #548 for more info.

Related PR's:

• xmtp openmls: Remove test only distinction for group_context_ext_proposal openmls#22
• proto: Add group mutable metadata for group names proto#158
• Part 1: Mutable Metadata group name Part 1 #643

Before merging this to main:

• Update openmls reference in Cargo.toml to commit on xmtp/openmls main branch
• Update dev/gen_protos.sh to point to xmtp-proto repo main branch

[insipx]

Something cool that nick found was using graphite to manage prs in parts like this, found it super useful so far in case you're interested, graphite