MetaBackend object for loading and listing backends

[BrunoLiegiBastonLiegi]

This PR implements a MetaBackend object that will call the different projects' (qibojit, qibolab, etc...) MetaBackend to load their backends. This will replace the current construct_backend function and will also give access to a list_available() function to list all the backends available in the system.

Closes #1096.

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.