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

# What

With the introduction of changes like support for [different encoding types of builder submissions](https://github.com/flashbots/relay-specs/issues/14) 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](https://ethereum.github.io/beacon-APIs/#/Config/getForkSchedule)

Start by reading issue #14 and the linked beacon-chain API examples for the config and node namespaces; the payload names no implementation files or tests. Done means agreeing on a relay configuration endpoint and incorporating its adopted contract into the relay API specification.

Written by the indexing model from the issue text.