[avalanche] Create a type alias for proof id sets

Summary:
Passing around sets of proof ids is becoming more common with changes like D12013. Providing a type alias
for this just makes sense.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12014