ext/{dbapi,pymysql}: Implement methods to (un)-instrument connections (…

…#624)

Implement to helper methods to allow users to enable / disable instrumentation
in a single connection object.

The current integrations based on dbapi allow to patch the connect function on those libraries to return an instrumented connection, however it's not possible to enable instrumentation on a connection previously created or to disable instrumentation in an instrumented connection.

This only implements the logic in PyMySQL, it can be easily done in other instrumentations once we think it's worth to be done.

Co-authored-by: Diego Hurtado <[email protected]>
Co-authored-by: Yusuke Tsutsumi <[email protected]>