CogRPC: Consider using binary metadata for CCA instead of text-based Authorization metadata

[gnarea]

The CCA is currently specified in the Authorization header/metadata because it belongs there from a semantic perspective, but we're having to serialise the CCA in base64 so we can use it in that text-based metadata.

By contrast, if we take a more pragmatic, performance-focused view, we should be using a binary metadata value, which would in turn force us to use a custom field.

This is a minor consideration in the large scheme of things but I thought it'd still be good to track it somewhere.