feat(signers): adds alloy-signer-gcp

[georgewhewell]

Motivation

Closes #71

Solution

upstreams https://github.com/georgewhewell/ethers-gcp-kms-signer

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes

[prestwich]

check my understanding:

• the GcpKeyRingRef specifies a keyring by project, location, and name
• it is extended with a key_id and a version to form a full key specifier
• that key specifier is passed in the requests

we may want to clean it up with something like this?

struct KeySpecifier { keyring: GcpKeyRingRef, key_id: String, version: u64 }

impl KeySpecifier { 
    fn to_key_version_ref(&self) -> String {
        // current behavior of `GcpKeyRingRef::to_key_version_ref`
    }
}

[georgewhewell]

yeah, that matches my understanding- eventually all these parameters are just used to construct a single string so have implemented KeySpecifier(String) since the inputs are never used individually

[gakonst]

@DaniPopes whenever you have time let's try to get this over the line!

[DaniPopes]

Thanks!
FYI I've edited the description to use GitHub keywords when referencing issues.