Restore CORS on git smart http protocol

[zeripath]

Unfortunately the chi changes have resulted in the CORS headers for the
git smart http protocol going missing.

This is mostly because the OPTIONS method is not being handled by
httpBase anymore.

This PR adds a GetOptions, PostOptions and Options methods to web
handler to allow OPTIONS method requests to still reach the httpBase
function.

Fix #16350
Close #16491

Signed-off-by: Andrew Thornton art27@cantab.net

[snowyu]

Confirmed, it works now.

[codecov-commenter]

Codecov Report

Merging #16496 (01387af) into main (49bd9a1) will increase coverage by 0.01%.
The diff coverage is 86.36%.

@@            Coverage Diff             @@
##             main   #16496      +/-   ##
==========================================
+ Coverage   45.49%   45.50%   +0.01%     
==========================================
  Files         718      718              
  Lines       84251    84262      +11     
==========================================
+ Hits        38327    38346      +19     
+ Misses      39762    39753       -9     
- Partials     6162     6163       +1     

Impacted Files	Coverage Δ
modules/web/route.go	80.50% <72.72%> (-0.46%)	⬇️
routers/web/web.go	91.73% <100.00%> (ø)
modules/charset/charset.go	71.71% <0.00%> (-4.05%)	⬇️
modules/queue/workerpool.go	53.05% <0.00%> (-1.53%)	⬇️
routers/web/repo/view.go	41.52% <0.00%> (+0.57%)	⬆️
models/repo_list.go	77.82% <0.00%> (+0.77%)	⬆️
modules/git/blame.go	67.14% <0.00%> (+1.42%)	⬆️
services/pull/patch.go	55.93% <0.00%> (+1.69%)	⬆️
models/unit.go	43.83% <0.00%> (+2.73%)	⬆️
services/pull/check.go	28.27% <0.00%> (+2.75%)	⬆️
... and 1 more

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[lunny]

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