hooks: Add support for gRPC mTLS

[TaridaGeorge]

This PR adds the following things:

1. Add the Authorization Header to the gRPC OutgoingContext in order to make use of it directly from the gRPC headers.
2. adds mTLS support for gRPC hook. grpc hook now have a hooks-grpc-secure flag

--hooks-grpc-secure=true
--hooks-grpc-server-tls-certificate=/home/user/tls.pem
--hooks-grpc-client-tls-certificate=/home/user/client.pem
--hooks-grpc-client-tls-key=/home/user/client_key.pem

[Acconut]

Thank you for the PR! These two changes should probably go into two separate PRs as they are distinct from each other.

1. Add the Authorization Header to the gRPC OutgoingContext in order to make use of it directly from the gRPC headers.

This should not be enabled by default as it allows a user to smuggle an arbitrary header value to the gRPC backend, which can have unintended consequences. Header forwarding should be an opt-in feature and even then be customizable to allow the administrator to select headers to be forwarded. Overall, I am thinking about an option that is similar to the -hooks-http-forward-headers flag that exists for HTTP hooks.

[Acconut]

I took the liberty to remove the Authorization header from this PR to get this merged. I'll open a new PR for implementing header forwarding via an option.

Thank you for working on this!

[Acconut]

PR for forwarding headers: #1188