Introduce scheduled pipelines for the non default external RPC process

This MR aims to bring the external rpc server in the CI's tezt tests.

First, Tezt looks for an environment variable that might be set to true during this scheduled pipeline. This env. var. is set when the dedicated scheduled pipeline is launched.

To do so, a scheduled pipeline was created: see "Automated external rpc process test" https://gitlab.com/tezos/tezos/-/pipeline_schedules

It is configured that way:

This schedule pipeline is derived from the "extended tests for master" which already exists since a while and aims to be run once a day, at midnight.

Manually testing the MR

Check that the scheduled pipeline is launch every day at midnight or check that ▶ can run the pipeline.

For now, the result of the pipeline can be a failure. Getting a green pipeline is not in the scope of this MR. Here we are just interested in automating the thing.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR