Migrate connections protocol to a plugin

[dbluhm]

This PR lifts the connections protocol out of ACA-Py proper and turns it into a plugin. The connections protocol has been deprecated for several releases. This completes the deprecation period while also encouraging the community to move on to bigger and better things (DID Exchange). But, for those who absolutely cannot move on, this plugin will fill the gap.

This is the first instance of a deprecated protocol entering retirement as a plugin. There may be processes to figure out still but I think it's time to make the jump.

Opening as draft while testing for input and thoughts.

cc @swcurran @jamshale

[dbluhm]

A corresponding PR will be opened to ACA-Py to remove the connections protocol.

[dbluhm]

Functionality lost by moving connections protocol to a plugin (e.g. things we may wish to reinstate as a feature of this plugin, if there's enough desire for it):

• Automatic mediator setup on startup using a connections invitation
• Automatic endorser setup on startup using a connections invitation

Things currently broken as a result of moving to a plugin (e.g. things we must fix to merge and have "good enough" feature parity):

• _fetch_connection_targets_for_invitation needs to be reworked or extended by plugin to support conn invite; consider mapping connections invitation into InvitationMessage for uniform handling.
• invitation attachment and retrieval; consider storing and retrieving using interface other than conn record
• request attachment and retrieval; consider storing and retrieving using interface other than conn record
• OOB Manager _perform_handshake

[dbluhm]

Corresponding PR in ACA-Py: hyperledger/aries-cloudagent-python#3184

[dbluhm]

This plugin is dependent on changes in an unreleased version of ACA-Py but is otherwise ready for review.