Keypair.SignData should except a Context parameter

[bdehamer]

Description

When building a Keypair implementation backed by a cloud HSM, it would be helpful if the SignData function accepted a Context for the current request.

This would allow for better retry/timeout support when interacting with the cloud HSM API.

Something like:

SignData(ctx context.Context, data []byte) ([]byte, []byte, error)

instead of the existing:

SignData(data []byte) ([]byte, []byte, error)