Add properties in QibolabBackend

[csookim]

Qibo needs qubits, connectivity, and natives to create a default transpiler.

https://github.com/qiboteam/qibo/blob/65bdfee7f5d30eb1b18cb31e54a28bac5bfd2e1a/src/qibo/backends/abstract.py#L32-L50

For native_gates, I returned all the native gates that can be executed on at least one case... (couplers are excluded) Please let me know if there is a better approach.

Checklist:

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.89%. Comparing base (a473d37) to head (a30c552).
Report is 18 commits behind head on 0.1.

Additional details and impacted files

@@            Coverage Diff             @@
##              0.1    #1076      +/-   ##
==========================================
+ Coverage   69.82%   69.89%   +0.07%     
==========================================
  Files          64       64              
  Lines        6859     6876      +17     
==========================================
+ Hits         4789     4806      +17     
  Misses       2070     2070              

Flag	Coverage Δ
unittests	69.89% <100.00%> (+0.07%)	⬆️

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[stavros11]

Thanks for the updates, it looks good to me now.