auth-server: Use api keys to authenticate proxy requests

[joeykraut]

Purpose

This PR uses the allocated API keys to authenticate an proxy request to the auth server. Specifically the flow is:

• Retrieve the user's encrypted API secret from the database using their API key as a key
• Decrypt the API secret using the server's decryption key
• Check an expiring HMAC on the request using the decrypted API secret
• If this passes, forward the request to the relayer

Todo

• Check the path to ensure it's an allowed path
• Attach admin auth to the request

Testing

• Built a client and allocated an API key for the client, verified that the client was able to call relayer methods through the auth server