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

tests-unit/README.md

@@ -0,0 +1,8 @@
+# Pytest Unit Tests
+
+## Install test dependencies
+
+`pip install -r tests-units/requirements.txt`
+
+## Run tests
+`pytest tests-units/`