Derive default auth data from the builder hostname

[nflaig]

The default data is currently the UTF-8 bytes of the builder URL exactly as advertised, so a trailing /, an explicit :443 or different casing in the proposer's configuration produces different signed bytes and fails the builder's check with a 400. This already happened on devnet-8 with a URL that only differed by a trailing slash, see the discussion on the Eth R&D Discord (#interop) and JasonVranek/epbs-config-flow#1 (comment).

This defines the default as the builder's hostname instead, lowercased ASCII with IPv6 literals in RFC 5952 form, dropping scheme, userinfo, port, path, query and fragment. It is a pure function of the configured URL string, so validator clients in mixed DV clusters derive identical bytes without a URL parser that implements WHATWG semantics, and it is applied when constructing the BuilderRequestAuth only, the signed bytes are still verified unchanged which addresses the concern that got the previous canonicalization removed.

• add a Default auth data section to the validator spec with the derivation, a reference function and test vectors
• reference the default from both builder checks and the BuilderRequestAuth.data description
• note that a builder needing a finer identity than its hostname agrees data with proposers out of band

Opened against the ethereum#165 branch as the auth data only exists there, keymanager-APIs ethereum#88 and beacon-APIs #630 restate the old default and will be updated to reference this section.

[nflaig]

Opened against the wrong repo by mistake, moved to ethereum#168

[nflaig]

sorry Jason, my claude is going crazy 😂