feat(signatures): allow using alternate google-kms client implementation

[beaulac]

google-cloud-node libraries are a notorious pain to use on serverless runtimes (e.g. CloudFlare Workers).

Importing @google-cloud/kms regardless of whether the client was provided or not brings in the whole google-cloud ecosystem which fails at dynamic-import-time due to various runtime incompatibility reasons (in our case, google-gax relying on __dirname).

This small change adds flexibility by

• only importing google-cloud libraries if the client was not provided
• narrowing the interface required for the client passed to the Signer, so that alternate client implementations don't need to stub unused methods

[vercel]

@beaulac is attempting to deploy a commit to the mythie's projects Team on Vercel.

A member of the Team first needs to authorize it.

[Mythie]

LGTM