Added none backend

[marcoSanti]

This pull request aims to improve the CAPIO backend component through the following changes:

• Relocation of MPI Logic: Moved MPI_Comm_size from the capio_server main function into the MPIBackend class constructor to improve encapsulation.
• Architecture Refactor: Performed a general refactor of the CAPIO backend by splitting class definitions and implementations into separate files.
• New NoBackend Class: Introduced a NoBackend class that performs no operations by default. This is intended for use cases requiring a single-node instance of the Capio server.
• Encapsulation of Global Variables: Removed the n_servers and node_name global variables, moving them into the Backend base class as private members.
• Removed inline Backend: Removed inline global declaration of Backend instance and moved to extern definition
• Dedicated Test Fixture with MockBackend in CapioFile: Created a dedicated Test Fixture with NoBackend instantiation for all the tests within the CapioFile unit tests that requires them

[codecov]

Codecov Report

❌ Patch coverage is 17.12329% with 121 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.79%. Comparing base (f3f8b10) to head (5924a3f).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
capio/server/src/mpi_backend.cpp	0.00%	27 Missing and 55 partials ⚠️
capio/server/src/remote_request.cpp	0.00%	9 Missing and 4 partials ⚠️
capio/server/src/none_backend.cpp	25.00%	4 Missing and 5 partials ⚠️
capio/server/include/remote/listener.hpp	50.00%	1 Missing and 5 partials ⚠️
capio/server/include/remote/requests.hpp	0.00%	1 Missing and 2 partials ⚠️
capio/server/include/handlers/getdents.hpp	0.00%	0 Missing and 2 partials ⚠️
capio/server/include/handlers/read.hpp	0.00%	0 Missing and 2 partials ⚠️
capio/server/include/handlers/stat.hpp	0.00%	0 Missing and 2 partials ⚠️
capio/server/include/remote/backend/none.hpp	0.00%	1 Missing ⚠️
capio/server/src/backend.cpp	92.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #217      +/-   ##
==========================================
- Coverage   64.70%   63.79%   -0.92%     
==========================================
  Files          73       77       +4     
  Lines        3165     3193      +28     
  Branches     1966     1988      +22     
==========================================
- Hits         2048     2037      -11     
- Misses        370      380      +10     
- Partials      747      776      +29     

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[GlassOfWhiskey]

LGTM