cli: Use PyMySQL for macOS MySQL compatibility

Fastpath MySQL transactions failed on macOS as mysqlclient tried to
load native '.so' plugin files. These files may not be available in
MySQL installations via Homebrew, which caused
MySQLdb.OperationalError during `fastpath result list/show` commands.

To resolve this, the PyMySQL module is now used as the MySQL driver.
PyMySQL is compatible with SQLAlchemy when used via monkey-patching
with `pymysql.install_as_MySQLdb()`.

Changes:
- Add pymysql to Python requirements.txt
- Patch resultstore in fastpath to use PyMySQL
- Remove default-libmysqlclient-dev from Dockerfiles

Signed-off-by: Aishwarya Rambhadran <aishwarya.rambhadran@arm.com>