Add support for starting the server on a unix domain sockets

[healthwaite]

Some folks may want the ability to start the rate limiter on a unix domain socket rather than a TCP/IP endpoint. This PR adds support for that.

• introduce a new environment variable GRPC_UDS
• if this variable is set we'll attempt to start the gRPC server on the unix domain socket address specified, otherwise we'll default to starting on GRPC_HOST:GRPC_PORT.

Example startup logs:

time="2024-03-20T11:10:23Z" level=info msg="Tracing disabled"
time="2024-03-20T11:10:23Z" level=warning msg="connecting to redis on redis-cluster-headless:6379 with pool size 10"
time="2024-03-20T11:10:23Z" level=warning msg="Creating cluster with urls [redis-cluster-headless:6379]"
time="2024-03-20T11:10:23Z" level=info msg="Waiting for initial ratelimit config update event"
time="2024-03-20T11:10:23Z" level=info msg="Successfully loaded new configuration"
time="2024-03-20T11:10:23Z" level=info msg="Successfully loaded the initial ratelimit configs"
time="2024-03-20T11:10:23Z" level=warning msg="Listening for HTTP on '0.0.0.0:8080'"
time="2024-03-20T11:10:23Z" level=warning msg="Listening for gRPC on '/tmp/grpc.sock'"
time="2024-03-20T11:10:23Z" level=warning msg="Listening for debug on '0.0.0.0:6070'"

I didn't see an obvious place to extend the unit tests.

[healthwaite]

@envoyproxy/ratelimit-maintainers

[ysawa0]

@healthwaite can you add how to use this to the documentation

[healthwaite]

@healthwaite can you add how to use this to the documentation

Sure - done

[healthwaite]

I've rebased and force pushed because I forgot to add a signature to my commits