feat: make corehttp a reusable component

[iand]

This makes the minimal changes needed to allow the corehttp package to be imported and reused by external applications.

Fixes #8524

It adds a new interface that defines the minimal set of methods needed to run corehttp as a gateway and is a non-breaking change.

As an example of use, where s.Gateway implements the four methods from the new interface

func (s *Server) registerHandlers() error {
	s.mux = http.NewServeMux()

	headers := make(map[string][]string)
	corehttp.AddAccessControlHeaders(headers)

	cfg := corehttp.GatewayConfig{
		Writable: false,
		Headers:  headers,
	}

	gwHandler := corehttp.NewGatewayHandler(cfg, s.Gateway)
	s.mux.Handle("/ipfs/", gwHandler)
	s.mux.Handle("/ipns/", gwHandler)

	return nil
}

(From https://github.com/iand/meridian/blob/006cf437183d07005e7b6254cb5a551eb64b102a/server.go#L77:L93)

[aschmahmann]

Looks great! Is a minimal change. Hopefully we can shrink that interface and get it even more minimal in the future.

---

There are some failing sharness tests here related to the gateway and seem related to your changes.

The failing tests are in test/sharness/t0117-gateway-block.sh.

expecting success: 
  grep "< Access-Control-Allow-Headers: Range" curl_output &&
  grep "< Access-Control-Allow-Headers: X-Custom1" curl_output &&
  grep "< Access-Control-Expose-Headers: Content-Range" curl_output &&
  grep "< Access-Control-Expose-Headers: Content-Length" curl_output &&
  grep "< Access-Control-Expose-Headers: X-Ipfs-Path" curl_output &&
  grep "< Access-Control-Expose-Headers: X-Ipfs-Roots" curl_output &&
  grep "< Access-Control-Expose-Headers: X-Custom2" curl_output

< Access-Control-Allow-Headers: Range

not ok 23 - Access-Control-Allow-Headers extends
#	
#	  grep "< Access-Control-Allow-Headers: Range" curl_output &&
#	  grep "< Access-Control-Allow-Headers: X-Custom1" curl_output &&
#	  grep "< Access-Control-Expose-Headers: Content-Range" curl_output &&
#	  grep "< Access-Control-Expose-Headers: Content-Length" curl_output &&
#	  grep "< Access-Control-Expose-Headers: X-Ipfs-Path" curl_output &&
#	  grep "< Access-Control-Expose-Headers: X-Ipfs-Roots" curl_output &&
#	  grep "< Access-Control-Expose-Headers: X-Custom2" curl_output
#	

expecting success: 
  grep "< Access-Control-Allow-Origin: localhost" curl_output

not ok 24 - Access-Control-Allow-Origin replaces
#	
#	  grep "< Access-Control-Allow-Origin: localhost" curl_output
#	

[iand]

@aschmahmann fixed header issue, PTAL

[iand]

Updated to incorporate changes from #9082

[BigLep]

@iand : we are hoping to land this for the 0.15 RC that we're cutting on Wednesday, 2022-08-17. Are you able to finish responding to comments so we can land this?

[iand]

@iand : we are hoping to land this for the 0.15 RC that we're cutting on Wednesday, 2022-08-17. Are you able to finish responding to comments so we can land this?

Done.

[lidel]

Thank you, @iand.

This is a good step towards #8524 (comment), and the changed surface is small enough to be safe to be included in 0.15-rc1