PTFE-1144 gcloud instances support of fixed labels

[tcarmet]

This pull request includes changes related to the configuration and testing of custom labels for GCP instances in the runner manager backend. The most important changes include adding a new attribute to the GCPInstanceConfig class to configure custom labels, and modifying the GCP backend test to verify the correct setting of custom labels.

Main configuration and testing changes:

• runner_manager/models/backend.py: Added a new attribute labels to the GCPInstanceConfig class to allow for the configuration of custom labels for GCP instances.
• tests/unit/backend/test_gcp.py: Added a new assertion to verify that custom labels are correctly set in the setup_labels method of the GCP backend, and modified the gcp_group method to ensure that a custom label is set for the GCP backend in the test. [1] [2]

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (195e920) 85.44% compared to head (923ed66) 85.45%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #496      +/-   ##
==========================================
+ Coverage   85.44%   85.45%   +0.01%     
==========================================
  Files          31       31              
  Lines        1168     1169       +1     
==========================================
+ Hits          998      999       +1     
  Misses        170      170              

Flag	Coverage Δ
api	66.12% <50.00%> (+0.02%)	⬆️
unit	71.08% <100.00%> (+0.02%)	⬆️

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