Provide overload type hints for search_issues variants

[Stealthii]

search_issues can return either a dictionary (if json_result) or a list of Issues under default behavior.

This change provides separate hints using overload so that type checked code doesn't need to cast unnecessarily when dealing with output.

Tests have been added, and cases where casting was used have been removed.

Fixes #1608.

[Stealthii]

The cloud / py3.8 job seems to be failing for all PRs, unrelated to the changes provided here.

[filbranden]

Looks awesome! Would be nice if this was merged, I run into issues having to cast this every time I use it