Family of Client Id (FoCI) support

[rayluo]

This PR contains:

• Refactor the cache test code, to provide build_response(...) helpers. They are helpful for the rest of this PR, and probably for all the recent and future PRs in unit testing.
• The main FoCI behaviors are implemented in 2 files:
  • application.py, its main logic closely represents the upper half of this internal diagram (extracted from this internal specs). We do not need to explicitly implement the bottom half of the diagram, because those behaviors already exist in our code base.
  • test_application.py, which is basically equivalent to this test file in MSAL .Net FOCI PR
• As a pre-existing behavior, the get_accounts() API does not filter account by client_id. This PR does not change that.
• HIstorically we do not yet have a remove_account() API (there is actually an open github issue/feature request for that). We will tackle that task at a later time, after some internal discussion/clarification on that topic settles down. For now, this PR contains no remove_account().

[henrik-me]

remove_account() : You can still add this using the behavior known for now. Some details may change later but that is ok. tracking it as a separate item is ok, but it should be part of the API even if the internal behavior may change. This is still in preview.

[henrik-me]

[abhidnya13]

LGTM