chunk large validator registrations

[tersec]

Otherwise large nodes might see, e.g.,:

{"lvl":"DBG","ts":"2022-11-26 19:26:36.267+00:00","msg":"REST request body has been sent","remote":"builder-relay-goerli.flashbots.net:443","request":"/eth/v1/builder/validators","size":3467933,"http_method":"POST","connection":"5"}
{"lvl":"DBG","ts":"2022-11-26 19:26:36.749+00:00","msg":"Got REST response headers from remote server","status":413,"http_method":"POST","remote":"builder-relay-goerli.flashbots.net:443","request":"/eth/v1/builder/validators","connection":"5"}
{"lvl":"DBG","ts":"2022-11-26 19:26:36.899+00:00","msg":"Received REST response body from remote server","remote":"builder-relay-goerli.flashbots.net:443","request":"/eth/v1/builder/validators","contentType":"text/html","size":192,"connection":"5"}

Builder API specs don't specify required allowed payload size, and there's a tradeoff between DoS-protection-trigger-risk and too-large-request-header risk, so try 1000 until more solid information comes in.