RFR 8223053: [xmldsig] Add KeyValue::EC_TYPE

[Weijun Wang]

Please take a review at the CSR at

   https://bugs.openjdk.java.net/browse/JDK-8223682

The text is copied from 4.5.2 and 4.5.2.3 of https://www.w3.org/TR/xmldsig-core/.

One thing I am not sure is that I haven't include the definition of ECParameters in 4.5.2.3.1 which is quite long. While I added an @implNote saying ECParameters is not supported in this implementation, I understand @implNote is not really a part of the spec  and it's not the reason I omit the definition of ECParameters. If you believe it should be included, I'll add it, or add a link.

The code change is exactly the same as the specification, so no webrev.

Thanks,
Max