feat(builder-spec): Correctly reject SSZ Accept/Content-Types headers.

[ryanschneider]

📝 Summary

As outlined in #677 this PR implements "Milestone 1" where we correctly reject SSZ Accept and Content-Type headers.

It adds two new helper functions, both of which depend on the new mimeheader dependency:

• RequestAcceptsJSON returns true if the request either does not include an Accept header or the header allows JSON response types.
• NegotiateRequestResponseType is not yet used, but shows how we can handle accept header parsing in Milestone 2.

⛱ Motivation and Context

Begins supporting ethereum/builder-specs#104 by implementing the first milestone in #677.

📚 References

---

✅ I have run these commands

• make lint
• make test-race
• go mod tidy
• I have seen and agree to CONTRIBUTING.md

[jtraglia]

Nice, this looks good. We should do this for registerValidators too. To preempt this:

• Add SSZ support to register validator api ethereum/builder-specs#110

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 62.50000% with 15 lines in your changes missing coverage. Please review.

Project coverage is 30.49%. Comparing base (13160be) to head (99062c7).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
services/api/service.go	62.50%	14 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #678      +/-   ##
==========================================
- Coverage   32.57%   30.49%   -2.09%     
==========================================
  Files          42       42              
  Lines        6972     7015      +43     
==========================================
- Hits         2271     2139     -132     
- Misses       4412     4602     +190     
+ Partials      289      274      -15     

Flag	Coverage Δ
unittests	30.49% <62.50%> (-2.09%)	⬇️

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[jtraglia]

LGTM, thanks @ryanschneider!