ref(integrations): add abstract search_issues method for issue integrations

[cathteng]

Define an abstract search_issues method in IssueBasicIntegration. This is used across Jira and SCM integrations -- am currently unsure of the return type, but this unifies the expectation of having issue search functionality across these integrations.

There are also search_issues functions in the integration API clients, but we should have the entry point be in installation.search_issues().

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 78.26087% with 10 lines in your changes missing coverage. Please review.

:white_check_mark: All tests successful. No failed tests found.

Files	Patch %	Lines
src/sentry/integrations/vsts/issues.py	22.22%	7 Missing :warning:
src/sentry/integrations/vsts/search.py	33.33%	2 Missing :warning:
src/sentry/integrations/example/integration.py	50.00%	1 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #76311       +/-   ##
===========================================
+ Coverage   66.99%   78.18%   +11.18%     
===========================================
  Files        6896     6901        +5     
  Lines      306738   307034      +296     
  Branches    52827    52840       +13     
===========================================
+ Hits       205512   240040    +34528     
+ Misses      94324    60594    -33730     
+ Partials     6902     6400      -502