add flag for mainnet forking

[LHerskind]

For better support with portals, it can be very useful to fork mainnet - as any L1 contract can then be used.

This require that anvil is setup to fork, but also that the archiver is told to sync from the same block as anvil forks unless you want to perform a sync from 0.

This should be possible by adding a rpc-url to the aztec-sandbox run command and set the SEARCH_START_BLOCK env variable for the archiver config.

Should either pull the current block for the start of the search or use a provided block number argument to both forking and start block.