Remove relay-metadata security schemes, expose the values in processing steps

[bbenligiray]

We have some security schemes named relay... that forwards RRP request metadata to the API for it to implement custom authentication schemes https://docs.api3.org/reference/airnode/latest/concepts/relay-meta-auth.html

• These clutter OIS with very protocol version-specific stuff
• It's not clear to handle how they behave if the request is does not belong to RRP. For example, it would be nice to be able to signal to the API that the request is coming from a call to the OEV gateway and not an RRP request.

Instead, we could just expose a metadata object that has all the required information to processing steps (similar to endpointParameters in #1673 (comment)) and expect the integrator to do what they need in preprocessing. After doing this, we can remove all metadata security schemes.