[Mypy] Add `src/snowflake/snowpark/_internal` to mypy coverage

[sfc-gh-stan]

Please answer these questions before submitting your pull requests. Thanks!

1. What GitHub issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   Fixes #NNNN

2. Fill out the following pre-review checklist:

   • [ ] I am adding a new automated test(s) to verify correctness of my new code
   • [ ] I am adding new logging messages
   • [ ] I am adding a new telemetry message
   • [ ] I am adding new credentials
   • [x] I am adding a new dependency

3. Please describe how your code solves the related issue.

   This PR includes the following changes:

• Add a mypy merge gate in Github Actions
• Add mypy configuration mypy.ini
• Fix type hints insrc/snowflake/snowpark/_internal