Allow rsa.PSSSaltLengthEqualsHash as option

[maraino]

This commit adds support to the salt length set to rsa.PSSSaltLengthEqualsHash when signing with an RSA key. This is currently supported, but the user must specify the length of the digest size.

Adding rsa.PSSSaltLengthEqualsHash allows the use of a key generated by the TPM for TLS client/server authentication. Go will set this option by default if an RSA key is used.

The client code looks like this:

if sigType == signatureRSAPSS {
        signOpts = &rsa.PSSOptions{SaltLength: rsa.PSSSaltLengthEqualsHash, Hash: sigHash}
}
sig, err := cert.PrivateKey.(crypto.Signer).Sign(c.config.rand(), signed, signOpts)