Support importing HMAC keys

[salrashid123]

FR to support importing HMAC key and using it via PKCS11, eg, with SoftHSM, i'd define something like the following to import an external HMAC key:

	hmacKeyTemplate := []*pkcs11.Attribute{
		pkcs11.NewAttribute(pkcs11.CKA_CLASS, pkcs11.CKO_SECRET_KEY),
		pkcs11.NewAttribute(pkcs11.CKA_KEY_TYPE, pkcs11.CKK_SHA256_HMAC),
		pkcs11.NewAttribute(pkcs11.CKA_SIGN, true),
		pkcs11.NewAttribute(pkcs11.CKA_VERIFY, true),
		pkcs11.NewAttribute(pkcs11.CKA_ENCRYPT, false),
		pkcs11.NewAttribute(pkcs11.CKA_DECRYPT, false),
		pkcs11.NewAttribute(pkcs11.CKA_WRAP, false),
		pkcs11.NewAttribute(pkcs11.CKA_UNWRAP, false),
		pkcs11.NewAttribute(pkcs11.CKA_TOKEN, true),
		pkcs11.NewAttribute(pkcs11.CKA_PRIVATE, true),
		pkcs11.NewAttribute(pkcs11.CKA_EXTRACTABLE, false), // we do need to extract this
		pkcs11.NewAttribute(pkcs11.CKA_SENSITIVE, true),
		pkcs11.NewAttribute(pkcs11.CKA_VALUE, []byte("change this password to a secret")), // make([]byte, 32)), /* KeyLength */
		pkcs11.NewAttribute(pkcs11.CKA_LABEL, "HMACKey"),                                  /* Name of Key */
		pkcs11.NewAttribute(pkcs11.CKA_ID, id),
	}

	hmacKey, err := p.CreateObject(session, hmacKeyTemplate)
	if err != nil {
		panic(fmt.Sprintf("GenerateKey() failed %s\n", err))
	}

---

depends on https://github.com/tpm2-software/tpm2-tools/issues/1597

[williamcroberts]

With some recent commits on master and PR #715 you can add a TPM generated HMAC key or import an existing HMAC key. You can even you tpm2-tools and link it in.

The one thing not supported, yet is doing it through PKCS11, so for now ill leave this open until that part is complete.