Added NoBackend

[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 17.00680% with 122 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.77%. Comparing base (f3f8b10) to head (0e36f13).

Files with missing lines	Patch %	Lines
capio/server/src/mpi_backend.cpp	0.00%	22 Missing and 48 partials ⚠️
capio/server/src/mpisync_backend.cpp	0.00%	5 Missing and 8 partials ⚠️
capio/server/src/remote_request.cpp	0.00%	9 Missing and 4 partials ⚠️
capio/server/src/no_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/no_backend.hpp	0.00%	1 Missing ⚠️
... and 1 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #217      +/-   ##
==========================================
- Coverage   64.70%   63.77%   -0.94%     
==========================================
  Files          73       78       +5     
  Lines        3165     3194      +29     
  Branches     1966     1989      +23     
==========================================
- Hits         2048     2037      -11     
- Misses        370      379       +9     
- Partials      747      778      +31     

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