[avalanche] Add a block registry and facility to register votes on these blocks.
This is essentially an interface to use what was created in D2040 in a way that is applicable to the Bitcoin Cash protocol.
Depends on D2040
Test Plan: Added test cases.
Reviewers: #bitcoin_abc, Fabien, schancel
Reviewed By: #bitcoin_abc, Fabien, schancel
Subscribers: teamcity, Fabien, jasonbcox, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2042