Add initial implementation of auth service

[adam-cattermole]

Changes

• Refactor of the ratelimit service and auth services into a generic GrpcServiceHandler
• Addition of the new HeaderResolver which is passed into each GrpcServiceHandler to ensure the headers are only retrieved once (and set in a OnceCell)
• For now the Auth and RL services are only used to create the messages and could be moved to an alternative location (for now auth marked as dead code until used)
• Fixed the integration test order of hostcalls and removed for the tests where there are no longer retrieval of headers

[didierofrivia]

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