Add getHeader SSZ support

[jtraglia]

📝 Summary

This PR adds SSZ support for getHeader endpoint. Please see:

• https://github.com/ethereum/builder-specs/blob/main/apis/builder/header.yaml

✅ I have run these commands

• make lint
• make test-race
• go mod tidy

[0xTylerHolmes]

I think they are fine, but two things that caught my attention:

1. Reusing a request object (it is fine if body == nil)
2. Double close scenarios (io.Closer specifies that calls to Close after first call is considered undefined; though I think it is fine here)

First thought that addresses both of these and simplifies things a bit is if we define a doRequest function inside get_header that we can use to handle both the SSZ and JSON request attempts:

doRequest := func(accept string) (*http.Response, error) {
                req, err := http.NewRequestWithContext(context.Background(), http.MethodGet, url, nil)
                if err != nil {
                    return nil, err
                }
                // <insert request prep>
                req.Header.Set("Accept", accept)
                return m.httpClientGetHeader.Do(req)
            }

Then we can do something like:

            // ssz attempt
            resp, err := doRequest("application/octet-stream")
            if err != nil {
                …
                return
            }
            // json if relay does not support ssz
            if resp.StatusCode == http.StatusNotAcceptable {
                resp.Body.Close()
                logRelay.Debug("SSZ not accepted, trying JSON")
                resp, err = doRequest("application/json")
                if err != nil {
                    …
		    return
                }
            }
            defer resp.Body.Close()
            // continue rest of execution
            ...

edit: get_header should probably use a context that we pass to the NewRequestWithContext so we can handle timeouts.