Define an API for DCs

[claucece]

Currently, there is a callback for using DC: GetDelegatedCredential. This can be improved to:

• Use the GetCertificate and GetCertificateClient callbacks to return the DCs as part of the Certificate structure
• Allow the possibility of returning a list of DCs: the appropriate DC will be chosen by using the advertise signature algorithm on the ClientHello/Certificate Request extension.
• Refactor the DC tests.

[claucece]

cc./ @armfazh @wbl

[claucece]

Related to #63

[cjpatton]

I like the idea of adding DCs to the crypto/tls.Certificate structure, since it allows us to expose DCs by whatever means is easiest to configure. Something like this?

type Certificate struct {
   ...
   DCs [][]byte
   DCPrivateKeys []crypto.PrivateKey
}

so that DCPrivateKeys[i] is the private key of DCs[i] for all 0 <= i <= len(DCs)?

[claucece]

@cjpatton check the latest pr: dc791fd

[cjpatton]

You're idea is much better than mine! I left some comments on the PR.

[claucece]

Merged now. Closing this.