Implement CORE & PROXY GRPC healthchecks

Right now there is no way to check if grpc is working correctly. There is a healthcheck spec for gRPC. Here is even example code: https://github.com/hyperium/tonic/tree/master/examples/src/health for tonic.

[x] implement GRPC healthcheck for CORE GRPC
[x] implement GRPC healthcheck for PROXY GRPC
[x] After implementation, please add to documentation information about helathhecks https://defguard.gitbook.io/defguard/admin-and-features/setting-up-your-instance/health-check
[x] Add to documentation examples how to check from command line (eg. curl) that proxy/core grpc is working

DefGuard / defguard