[avalanche] Add count of orphan proofs to getavalancheinfo

Summary:
This is useful for debugging and testing.

Depends on D11696

Test Plan:

ninja check check-functional
bitcoin-cli help getavalancheinfo

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D11697