Feature: Asymmetric Signatures for GCP KMS

[sgammon]

This changeset extends the existing KMS infrastructure for GCP to support the AsymmetricSignRequest/AsymmetricSignResponse flow.

Features so far:

• Ability to spawn a PublicKeySign backed by GCP KMS
• Ability to sign data (provided it is pre-hashed)
• Ability to fetch the public key and verify data
• Safer handling of input
• Unit testing

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[sgammon]

@googlebot I signed it!

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[juergw]

Sorry that we have not replied yet to this request.

We prefer not to merge this into the Tink library. The main goal of KmsClients is to have a way to encrypt and decrypt keysets. We currently don't want to extend that to anything else.