Make OPTIONS method on MSC3916 endpoints available without auth

[arenekosreal]

OPTIONS method is usually sent by browser in preflight requests, most of the time we cannot control preflight request to add auth header.

Synapse will return a 204 response directly without authentication for those OPTIONS method.

According to firefox's documentation, both 200 and 204 are acceptable so I think there is no need to change handler in dendrite.

This closes #3424

No need to add a test because this is just a fix and I have tested on my Cinny Web client personally.

Pull Request Checklist

• I have added Go unit tests or Complement integration tests for this PR or I have justified why this PR doesn't need tests
• Pull request includes a sign off below using a legally identifiable name or I have already signed off privately

Signed-off-by: arenekosreal <17194552+arenekosreal@users.noreply.github.com>

[S7evinK]

While this works, I'd prefer having the OPTIONS check in the MakeHTTPAPI function, i.e.

// MakeHTTPAPI adds Span metrics to the HTML Handler function
// This is used to serve HTML alongside JSON error messages
func MakeHTTPAPI(metricsName string, userAPI userapi.QueryAcccessTokenAPI, enableMetrics bool, f func(http.ResponseWriter, *http.Request), checks ...AuthAPIOption) http.Handler {
	withSpan := func(w http.ResponseWriter, req *http.Request) {
		if req.Method == http.MethodOptions {
			util.SetCORSHeaders(w)
			w.WriteHeader(http.StatusOK)
			return
		}
...

[arenekosreal]

I have changed the code to check OPTIONS in MakeHTTPAPI, I must say the diff is much cleaner than the old one.

[S7evinK]

Thanks!

[S7evinK]

We're using http.StatusOK in MakeJSONAPI, so I'd keep that the same.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 68.36%. Comparing base (f2db7cb) to head (29028d7).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/httputil/httpapi.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3431      +/-   ##
==========================================
+ Coverage   68.34%   68.36%   +0.02%     
==========================================
  Files         513      513              
  Lines       47026    47030       +4     
==========================================
+ Hits        32138    32153      +15     
+ Misses      10896    10886      -10     
+ Partials     3992     3991       -1     

Flag	Coverage Δ
unittests	53.35% <0.00%> (+0.03%)	⬆️

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