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

pytest.ini

@@ -1,5 +1,8 @@
 [pytest]
 markers = 
   inference: mark as inference test (deselect with '-m "not inference"')
-testpaths = tests
-addopts = -s
+testpaths =
+  tests
+  tests-unit
+addopts = -s
+pythonpath = .