### Motivation and Context
Currently, the Python connectors for Google AI and Vertex AI don't pass
the standard semantic-kernel user agent string. This PR adds the user
agent header so that the Google API calls properly identify Semantic
Kernel.
This aligns with the dotnet implementation, and our (Google DeepMind's)
[best
practices](https://ai.google.dev/gemini-api/docs/partner-integration).
### Description
Sends the `User-Agent` header conditionally in `GoogleAIChatCompletion`,
`GoogleAITextCompletion`, and `GoogleAITextEmbedding`, matching the
OpenAI implementation as much as possible (gated behind telemetry flag,
using `SEMANTIC_KERNEL_USER_AGENT`).
### Contribution Checklist
- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄.
