Restore CORS on git smart http protocol (#16496) (#16506)

Restore CORS on git smart http protocol (#16496) (#16506)

Backport #16496

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>