Add "testnet scenarios" as a generator in TeCI

This means adding a new generator (in the folder generators) which should be a Docker image containing the binary produced in #48. The entrypoint of the image should take the RPC endpoint of a node as argument, and run the scenario against the network of that node. I'm not sure how to ensure that the node launched by the testnet scenario connects to the network spawned by TeCI. It is possible that the network settings in https://gitlab.com/tezos/teci/blob/main/docker/sandbox_test_network/docker-compose-sandbox-autobake.yml#L13 must be modified.

Small WIP: !52 (closed)