Add EnvironmentVariables field in hook rpcs

Currently, the Hook RPCs (PreReceiveHook, UpdateHook, PostReceiveHook) do not pass in all of the gl_* values into the hooks.

To fix this, this MR adds a EnvironmentVariables field in each of the hook rpcs that can be used to pass in values for gl_id, gl_protocol, and gl_username so we can pass these values into the ruby hooks. gl_repository will be passed through the repository struct.

This MR also adds test coverage to ensure that the GL_USERNAME, GL_PROTOCOL, GL_ID, GL_REPOSITORY values get passed through to the hooks.

part of: #1701 (closed)