pcli: creating a validator template should accept cli args for field values

[conorsch]

To create a validator, our docs recommend running pcli validator definition template, and then manually editing the toml fields. We can preserve that default behavior, but we should also support scripting via cli args like --website, --name, etc. Otherwise, dev teams and operators must resort to e.g. sed, as in https://github.com/penumbra-zone/web/blob/74d9a8357db8371bc0002c3b652bd768d3e8b054/scripts/create-validators.sh#L47-L53

I also suggest that running pcli validator definition template without providing a governance pubkey string or filepath should be an error, not a warning, since that validator config cannot possibly work. To accommodate testing workflows, we can add a flag like --generate-ephemeral-governance-pubkey that preserves the behavior.