Expose isActive() method in Transaction interface

[b-admike]

Steps to reproduce

Coming from #3397 (comment). We need to expose an API for transactions in LB4 to check if a transaction is active or not.

Acceptance Criteria

• Create a isActive method in loopback-connector that checks if the connection object on a transaction is present or not, thus indicating if transaction itself is active. Add unit tests to verify the behaviour and release a new minor version of the module.
• Include the method definition in Transaction interface in loopback-next and use it in tests and before/after hooks to check if transactions are active

[jannyHou]

closing since all related PR merged. And see the postgresq follow-up story in loopbackio/loopback-connector-postgresql#411