filter: Add `maybe_add_search_terms` function to add search_term.
filter: Add `maybe_add_search_terms` function to add search_term.

This function adds search_term to the operators list.
This logic is extracted as a prep commit for the
changes related to #19435.

rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,kou/zulip,kou/zulip,andersk/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,rht/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,rht/zulip