Validator should be able to be configured to use GCP storage for checkpoints

[tkporter]

Problem

• GCP checkpoint storage is implemented and the relayer is seemingly capable of reading checkpoints
• Validator settings seem to not support this case though

Solution

• Update the match that validators use to support using GCP storage https://github.com/hyperlane-xyz/hyperlane-monorepo/blob/44cc9bf6b33f5288462ecc027e9d624ae682d552/rust/agents/validator/src/settings.rs#L151-L195
• Probably get rid of the Some(_) branch so this doesn't happen again
• Probably worth a run to confirm it actually works, and maybe another pass on the code

[mawutory]

@tkporter can i pick this up? this would be my first.