Support custom headers

[alexanderzobnin]

When openfga is embedded in another application as library, there might be different authentication method in front of GRPC connection. For example, in grafana we use token-based authentication for GRPC connection to the service which embeds openfga. So in order to use openfga-cli we need to pass access token in special header which is not possible currently with CLI.

[aaguiarz]

Thanks @alexanderzobnin ! Can you suggest a syntax that would work for you?

[alexanderzobnin]

I'm working on PR, so we can discuss it one it's ready.

[alexanderzobnin]

I suggested --custom-headers flag which allows to pass any required headers, so it's quite flexible.