SVIDs must be issued over gRPC

[jsnctl]

Per the SPIFFE standard https://github.com/spiffe/spiffe/blob/main/standards/SPIFFE_Workload_Endpoint.md#3-transport

The SPIFFE Workload Endpoint MUST be served over gRPC, and compliant clients MUST support gRPC. It may be exposed as either a Unix Domain Socket (UDS) or a TCP listen socket. Implementations SHOULD prefer Unix Domain Socket transport, however TCP is supported for implementations in which Unix Domain Sockets are impractical or impossible. TCP transport MUST NOT be used unless the underlying network allows the Workload Endpoint server to strongly authenticate the workload based on source IP address (e.g., over a localhost or link-local network), or other strong network-level assertions (e.g., via an SDN policy).

At present, kubespiffed has the v1/svid endpoint available over HTTP - this needs to at least be changed to gRPC (along with potential other amendments for standard compliance)