Add FrontendManager to manage non-default front-end impl (#3897)

* Add frontend manager

* Add tests

* nit

* Add unit test to github CI

* Fix path

* nit

* ignore

* Add logging

* Install test deps

* Remove 'stable' keyword support

* Update test

* Add web-root arg

* Rename web-root to front-end-root

* Add test on non-exist version number

* Use repo owner/name to replace hard coded provider list

* Inline cmd args

* nit

* Fix unit test

.gitignore

@@ -17,4 +17,5 @@ venv/
 !/web/extensions/core/
 /tests-ui/data/object_info.json
 /user/
-*.log
+*.log
+web_custom_versions/