Add user agent to telemetry

[nookala]

Thanks for contributing to cloud_controller_ng. To speed up the process of reviewing your pull request please provide us with:

• A short explanation of the proposed change:
  Add user agent to telemetry logs. This would be useful to get the cli versions that customers are running.

• An explanation of the use cases your change solves

Example with user agent in the telemetry logs

{
"telemetry-source": "cloud_controller_ng",
"telemetry-time": "2025-10-27T14:52:22+00:00",
"build-completed": {
"api-version": "internal",
"lifecycle": "buildpack",
"buildpacks": [
"staticfile_buildpack"
],
"stack": "cflinuxfs4",
"user-agent": "Go-http-client/1.1",
"app-id": "0e43f6e2738ecfe1244b5a8b334337a7d8cdcb0d88345d27f84c9b80fc04d1dc",
"build-id": "db2237a9d82f0450fd017d4782798a9e7a024bb527cd9d9f0367f3e31fa82e68"
}

• Links to any other associated PRs

• I have reviewed the contributing guide

• I have viewed, signed, and submitted the Contributor License Agreement

• I have made this pull request to the main branch

• I have run all the unit tests using bundle exec rake

• I have run CF Acceptance Tests