Client identification for Gemini API

[thorwebdev]

Hey team, would it be possible to add the following client identification to the library to help us track usage origins better?

X-Goog-Api-Client: API client identification. The value is a space-separated list of NAME "/" SEMVER strings, where the NAME should only contain lowercase letters, digits, and "-", and the SEMVER should be a semantic version string. For example: X-Goog-Api-Client: python/3.5.0 grpc-google-pubsub-v1/0.1.0-beta2 linux/2.7.0.

See system parameter docs here: https://docs.cloud.google.com/apis/docs/system-parameters

Example: 
## Python:

from google import genai

client = genai.Client(
    api_key="...",
    http_options={
        "headers": {
            "x-goog-api-client": "my-sdk-py/1.23",
        }
    }
)

## JavaScript
import { GoogleGenAI } from '@google/genai';

const ai = new GoogleGenAI({
    apiKey: '...',
    httpOptions: {
        headers: {
            'x-goog-api-client': 'my-sdk-js/1.23'
        }
    }
});

## Direct API
curl https://generativelanguage.googleapis.com/v1beta/models/gemini-2.5-flash:generateContent?key=$GOOGLE_API_KEY \
    -H 'Content-Type: application/json'
    -H 'X-Goog-Api-Client: my-sdk/1.23'
    -d '{...}'

You can see an example implementation of that client identification in the SDK here: https://github.com/google/adk-python/blob/main/src/google/adk/models/google_llm.py#L262-L272