feat: added the ability to set a default AI provider for serve

[yankay]

Closes #

📑 Description

The k8sgpt serve default backend is hardcoded as openai.
The PR is to add the ability to set a default AI provider for serve by configure the same as the command analysis

✅ Checks

• My pull request adheres to the code style of this project
• My code requires changes to the documentation
• I have updated the documentation as required
• All the tests have passed

ℹ Additional Information

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Project coverage is 35.82%. Comparing base (f9d734e) to head (9787881).
Report is 57 commits behind head on main.

Files	Patch %	Lines
cmd/serve/serve.go	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1216      +/-   ##
==========================================
- Coverage   37.27%   35.82%   -1.46%     
==========================================
  Files          83       87       +4     
  Lines        5615     6052     +437     
==========================================
+ Hits         2093     2168      +75     
- Misses       3442     3796     +354     
- Partials       80       88       +8     

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