feat: allow to use a different mnemonic

[leovct]

Using a different mnemonic is not supported for now because we rely on pre-generated validator accounts to make the deployment faster. In order to solve this problem, we would need to implement a new service that would create the validator accounts on the fly if the mnemonic is different than the default mnemonic.

For reference: https://github.com/0xPolygon/kurtosis-polygon-pos/blob/main/main.star#L242-L249

Note that this is the same problem if you set up a devnet with more validators than the numbers of pre-generated accounts (1000).