Send validator registrations as SSZ instead of JSON

[nflaig]

See ethereum/builder-specs#110

Right now we limit this api to send JSON-only

lodestar/packages/api/src/builder/routes.ts

Line 93 in 0ac8bb9

req: JsonOnlyReq({

Our HTTP client will already fallback to JSON if we receive a 415 status code but we will have to add custom retry logic for this specific call to consider any HTTP error (but not network errors) and if we encounter an error we should only send JSON afterwards.