[assumeutxo] add mainnet and testnet UTXO snapshot checkpoints

Summary:
Make it possible to use the loadtxoutset command with UTXO snapshots matching these exact parameters.

Depends on D18098

Test Plan:

src/bitcoin-cli  [-testnet] -rpcclienttimeout=0 -named dumptxoutset utxo_testnet_${height}.dat rollback=${height}

Check the output matches these parameters.

Move the utxo snapshot files out of the .bitcoin[/testnet3] data directory.
With a fresh node (new data directory), start the node then load the utxo snapshot and let it fully sync.

src/bitcoin-cli [-testnet] -rpcclienttimeout=0 loadtxoutset ~/utxo_mainnet_${height}.dat

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18093