JWT SVID support for Delegated Identity API

[loveyana]

Thanks to the outstanding work done by the community, we currently support the Delegated Identity API, so that spire has the delegated mode. Based on it, I did the design and development of spire-csi.

The main purpose of this issue is to add the JWT-SVID privilege api in the Delegated Identity API, so that privileged workload can fetch JWT-SVIDs and subscribe to JWKS bundles. In order to achieve this purpose, I created this PR spiffe/spire-api-sdk#20.

I would be happy to further discuss and implement the idea if it makes sense. Thanks!