[avalanche] Add the dangling proofs count to the getavalancheinfo RPC

Summary: Returns the count of dangling proofs (i.e. proofs with no node connected) as well as their associated stake amount.

Test Plan:

./test/functional/test_runner.py abc_rpc_getavalancheinfo.py

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11782