Add ETag header (#15370) (#15552)

Add ETag header (#15370) (#15552)

* Add ETag header.

* Comply with RFC 7232.

* Moved logic into httpcache.go

* Changed name.

* Lint

* Implemented If-None-Match list.

* Fixed missing header on *

* Removed weak etag support.

* Removed * support.

* Added unit test.

* Lint

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>