Add py.typed

[wouterdb]

We should add a py.typed file

However, this may cause issues

Know issues:

1. the result of regex_string is intended to be used as a type (for pydantic) but it is not typed as a type

inmanta_plugins/connect/configuration.py:626: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
inmanta_plugins/connect/configuration.py:637: error: Variable "inmanta_plugins.config.const.InmantaTemplatePath" is not valid as a type  [valid-type]

• This should be well coordinated with solutions team
• enabling mypy baseline on all solutions repos may be required
• If done, add similar ticket to lsm