feat: add user agent parameter for agent

[alswl]

For traffic control and log analysis, we use the UA to identify the client's identity.

This PR allows the user to set up the User Agent by adding a command line parameter, which facilitates tracking and analysis for performance evaluation.

Usage:

./agent --user-agent "argocd-application-controller" ...

[sonarqubecloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov[bot]]

Codecov Report

Base: 55.46% // Head: 55.46% // No change to project coverage :thumbsup:

Coverage data is based on head (650d39f) compared to base (98ccd3d). Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #472   +/-   ##
=======================================
  Coverage   55.46%   55.46%           
=======================================
  Files          41       41           
  Lines        4504     4504           
=======================================
  Hits         2498     2498           
  Misses       1813     1813           
  Partials      193      193           

Impacted Files	Coverage Δ
pkg/cache/cluster.go	54.86% <0.00%> (ø)

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[alswl]

@zachaller

Hi, could you review this simple pull request?

[alswl]

Friendly ping with @alexmt .