Expose per-invocation cost in LlmResponse (LiteLLM and other providers)

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[Contribution guide.](https://google.github.io/adk-docs/contributing-guide/)

**Is your feature request related to a problem? Please describe.**
ADK exposes token usage via `LlmResponse.usage_metadata` but does not expose per-call cost. With LiteLLM (and some other providers), a computed cost is available after each invocation (e.g., `completion_cost(response)` or `_hidden_params["response_cost"]`). Because ADK doesn’t surface this, applications must either (a) maintain price sheets and recompute cost from tokens (risking drift and inconsistency) or (b) patch adapters to pull provider cost directly.

**Describe the solution you'd like**
Add an optional, typed field on `LlmResponse` to standardize cost exposure across providers, e.g.:
- `LlmResponse.cost_metadata: Optional[CostMetadata]`
- `total_cost_usd: Optional[float]`
- `prompt_cost_usd: Optional[float]`
- `output_cost_usd: Optional[float]`
- `currency: Optional[str]` (default “USD” when known)
- `provider: Optional[str]` (e.g., “litellm”, “vertexai”, “openai”)
- `source: Optional[Literal['provider','adapter','computed']]`
- `raw: Optional[dict]` (provider-specific passthrough like `response_cost`)
Population strategy:
- LiteLLM:
- Non-streaming: prefer `litellm.completion_cost(response)`; fallback to `response._hidden_params['response_cost']` when present.
- Streaming: read from stream wrapper if exposed; otherwise leave unset (no silent recompute).
- Other providers:
- If SDK exposes cost, pass it through into `cost_metadata`; else `None`.

**Describe alternatives you've considered**
- Client-side recomputation using `usage_metadata` + custom price sheets: works, but requires price maintenance, risks drift, and yields provider-inconsistent results.
- Stuffing cost into `custom_metadata`: not discoverable/typed; harder to rely on across apps.

**Additional context**
- Suggested touch points:
- `src/google/adk/models/llm_response.py`: add `CostMetadata` and `cost_metadata`.
- `src/google/adk/models/lite_llm.py`: populate for non-streaming and streaming when available.
- (Optional) `plugins/logging_plugin.py`: log cost if present.
- (Optional) `telemetry/tracing.py`: emit span attribute (e.g., `gen_ai.cost.total_usd`).
- Example (illustrative):
```json
"cost_metadata": {
"total_cost_usd": 0.00123,
"prompt_cost_usd": 0.00040,
"output_cost_usd": 0.00083,
"currency": "USD",
"provider": "litellm",
"source": "provider",
"raw": { "response_cost": 0.00123 }
}
```