Replace builder_index with opaque bytes in RequestAuth

[JasonVranek]

We require per-builder authorization on GET /execution_payload_bid and POST /validators to prevent builder-to-builder replay attacks. The current PR changed the authorization data from a builder url to builder_index. A single builder_index is insufficient as it is likely there isn't a 1:1 relationship between a URLs and indices which will result in the validator needing to generate multiple signatures and make multiple redundant requests to the same URL. The same reasoning applies if builder_index is replaced by builder_pubkey.

The purpose of this PR is to future proof ourselves by generalizing the RequestAuth struct to:

class RequestAuth(Container):
    message: ByteList[MAX_AUTH_DATA_BYTES]

Having message as opaque bytes allows for arbitrary authorization schemes to be used in the future. For example message can encode a builder's URL or something more complicated, as long as the builder understands how to interpret the bytes.