allow specifying test size in gazelle_python_manifest

Relevant Rules

gazelle_python_manifest

Description

gazelle_python_manifest generates an sh_test.
In my case, bazel test complains that the specified timeout is too long:

WARNING: //:gazelle_python_manifest.test: Test execution time (0.2s excluding execution overhead) outside of range for MODERATE tests. Consider setting timeout="short" or size="small".

Describe the solution you'd like

A test_timeout parameter to gazelle_python_manifest.

Describe alternatives you've considered

Just ignoring the warning.

If this is acceptable I could try to contribute the PR.