Add count method to clients

[gmazoyer]

Fixes #158

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
infrahub_sdk/client.py	71.42%	2 Missing and 2 partials ⚠️

@@           Coverage Diff            @@
##           develop     #225   +/-   ##
========================================
  Coverage    68.03%   68.03%           
========================================
  Files           82       82           
  Lines         7370     7384   +14     
  Branches      1440     1442    +2     
========================================
+ Hits          5014     5024   +10     
- Misses        1984     1986    +2     
- Partials       372      374    +2     

Flag	Coverage Δ
integration-tests	12.98% <0.00%> (-0.03%)	⬇️
python-3.10	43.29% <57.14%> (+0.02%)	⬆️
python-3.11	43.29% <57.14%> (+0.02%)	⬆️
python-3.12	43.29% <57.14%> (+0.02%)	⬆️
python-3.13	43.29% <57.14%> (+0.02%)	⬆️
python-3.9	41.73% <57.14%> (+0.02%)	⬆️
python-filler-3.12	24.05% <14.28%> (-0.02%)	⬇️

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Files with missing lines	Coverage Δ
infrahub_sdk/client.py	64.07% <71.42%> (+0.12%)	⬆️