Adds a health checking endpoint on the GRPC server. This is a stream that changes whenever a health status update occurs.
Reviewed-on: https://git.nikos.gg/prymn/prymn/pulls/5
Co-authored-by: Nikos Papadakis <nikos@papadakis.xyz>
Co-committed-by: Nikos Papadakis <nikos@papadakis.xyz>