SQLAlchemyInstallationStore: Non-English column data support for MySQL use cases

[seratch]

As reported at slackapi/bolt-python#974, the built-in SQLAlchemyInstallationStore's table definition does not propertly support non-English character data when using it with MySQL / MariaDB.

Reproducible in:

See slackapi/bolt-python#974

The Slack SDK version

Any versions

Python runtime version

Any

OS info

Any