abci++: Define and implement pruning strategy for stored extended commits

[thanethomson]

As per RFC 017 (#8317), we are going to be storing all extended commits in the block store, which will contain vote extensions and vote extension signatures. This could take up a substantial amount of disk space for large vote extensions and large validator sets.

We need to define and implement an optimal pruning strategy in order to minimize the operational impact of storing extended commits while still maintaining overall network integrity.

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