models: Add `clear_client_cache` function to clear `get_client_cache`.
models: Add `clear_client_cache` function to clear `get_client_cache`.

This is a prep commit, this function will be used when we
want to reset the test environment of the puppeteer test.

punchagan/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,andersk/zulip,zulip/zulip,kou/zulip,eeshangarg/zulip,kou/zulip,hackerkid/zulip,punchagan/zulip,hackerkid/zulip,rht/zulip,kou/zulip,andersk/zulip,andersk/zulip,punchagan/zulip,hackerkid/zulip,kou/zulip,hackerkid/zulip,zulip/zulip,rht/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,hackerkid/zulip,punchagan/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,punchagan/zulip,andersk/zulip,andersk/zulip,rht/zulip,hackerkid/zulip,eeshangarg/zulip,punchagan/zulip,kou/zulip,punchagan/zulip,rht/zulip,hackerkid/zulip,rht/zulip,kou/zulip,rht/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip