[java-kms] Support integration as Java JCA provider

The Java JCE/JCA APIs are well established standards how to integrate cryptographic primitives into a Java application (see https://en.wikipedia.org/wiki/Java_Cryptography_Architecture). Accordingly a vendor adhering to those standards can easily be integrated into a wide range of products.

It would be great to see Google Cloud HSM/KMS to support those standards in the Java API. Other cloud vendors do similar things, see
https://stackoverflow.com/questions/53394842/google-cloud-hsm-as-a-provider-for-encryption. E.g. in Azure once has those connectors resp. also general pkcs 11 support.

As consequence, users currently have to write that integration on their own.