RFC: add a `SignerWithContext` interface for custom sign implementations

[jamestelfer]

The current sign interface has the single Sign method:

Sign(claims jwt.Claims) (string, error)

When implementing an AWS KMS signer, the lack of the context means the remote operation doesn't participate in the main context, so it's unaware of request timeouts or server shutdown.

What about adding:

type SignerWithContext interface {
    Sign(ctx context.Context, claims jwt.Claims) (string, error)
}

An existing Signer implementation can be supported through a simple wrapper function:

type SignerWithContextFunc func Sign(claims jwt.Claims) (string, error)

func (s SignerWithContextFunc) (_ context.Context, claims jwt.Claims) (string, error) {
    return s(claims)
}

// ... 

signer := // Signer implementation
contextSigner := SignerWithContextFunc(signer.Sign)

Then a configuration item for WithContextSigner or similar. Thoughts?

I am able to raise a PR for this, or some more acceptable variation.

[jamestelfer]

I've created a PR fleshing out the idea if you're interested @bradleyfalzon. Feedback welcome!

[jamestelfer]

Another valuable use case for the context is when using telemetry. The current trace is generally carried on the request context, so including the context allows actions taken during signing to be included in the wider request trace.