[avalanche] Make proof dust threshold configurable

Summary:
Bumping the dust threshold will unnecessarily complicate the functional tests
since it will require each proof-aware test to mine coinbases, mine more to
make those coinbases mature, then merge the coinbase utxos to at least the dust
threshold, then mine those txs to make the proof valid. While doable, the tests
will be unreasonably slow due to mining hundreds of blocks for each case.

The better solution is to make the dust threshold configurable so the tests need
only mine the coinbases necessary to test, and then skip the utxo management
steps. This saves minutes of test time for abc_rpc_avalancheproof alone due to
the too_many_stakes case.

Depends on D11749 and D11761

Test Plan:

ninja check bitcoind
bitcoind -avaproofstakeutxodustthreshold=0                # fails
bitcoind -avaproofstakeutxodustthreshold=-1               # fails
bitcoind -avaproofstakeutxodustthreshold=99999            # fails
bitcoind -avaproofstakeutxodustthreshold=99999 -testnet   # ok

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D11750