Support for TPM-backed signers

[getvictor]

I am using HTTP signatures with a TPM (Trusted Platform Module), where the private key for the algorithm is located in the hardware, and I must use system APIs for signing.

This change allows the usage of a custom crypto.Signer:

httpsig.SigningKey{
	Key:       myTPM.Signer(),
	MetaKeyID: keyID,
}

I have verified this as working on our project, and we plan to put these changes into production within the next month:
https://github.com/fleetdm/fleet/tree/victor/29935-tpm-agent

[getvictor]

@leelynne Can you take a look at this PR? Let me know if you have feedback/suggestions.

[getvictor]

@leelynne Do you know when you'll be able to take a look at this PR?

[leelynne]

@getvictor My apologies I need to get my github notifications in order! Taking a look now.

[leelynne]

Would be able to able to add a test for usage of crypto.Signer? If not, no worries I will add the funcitonality and test. I need to add a set of tests for sign anyway.

[getvictor]

Please add a test. I'm confident it works since we have integration tests using this library, and we'll be shipping it to production in ~2 weeks (after QA is done).

Also, I assume you will merge the PRs that you approved. I don't have permissions to merge them myself.

[leelynne]

Going to close this in favor of #32.

Note that the crypto.Signer compatible Sign method for ecdsa returns the ASN.1 encoding of the signature which is not compatible with the RFC. If you used a version of this code be sure to update the signature logic.