[Proposal] Update API specification to include endpoint for Relay Server configuration information

[Wazzymandias]

What

With the introduction of changes like support for different encoding types of builder submissions there can be value in exposing an endpoint that returns relay server information to a client. For example, let's say a builder wants to send SSZ payload to relay server, but relay server does not support SSZ because it's on older version. Rather than trying different payload encodings, the builder could query a relay endpoint to determine which format is acceptable.

Note: The example is for illustrative purposes only. Relay server configuration endpoint would first need to be adopted before it could be used to gauge relay behavior.

This idea is conceptually similar to config and node namespace endpoints defined in beacon chain API: example