Add xmtp_xid

[insipx]

• Adds a new crate for xmtp_xid closes Scaffold new identity crate in libxmtp #588
• did some general cleanup around using workspace dependencies in xmtp_mls and related crates

not really sure how much we need here so I just copied a very minimum of the Identity struct from xmtp_mls

We probably want to reduce dependency on xmtp_mls as much as possible so we don't have a circular dependency, which implies also importing GrantMessagingAccessAssociation. This may make more sense to be apart of #589 & the trait. We can potentially do something to the effect of:

• decouple the 'association text' and EOA signing stuff
• refactor messaging associations to be just EOA signature credential
• implement new trait from XIP for these structs
  This will be clearer for me once I play around with the trait in 589 a bit

[neekolas]

Agree we'll want to avoid a circular dependency. Think we can just copy as needed for now, and then only import from xmtp_mls and delete the xmtp_mls code once this is ready

[37ng]

Suggest to rename xmtp_xid to xmtp_id since x represents xmtp

[neekolas]

Sounds reasonable to me. It's weird enough that we are prefixing all of our stuff with the company name, we don't need to do it twice