Add usageMetadata to the LlmResponse

[Lezeper]

Is your feature request related to a problem? Please describe.
I need to capture token count details when constructing the LlmResponse in a custom model.

Describe the solution you'd like
Include a "usageMetadata" field in the LlmResponse.java class.

Describe alternatives you've considered
Without adding usageMetadata to LlmResponse, I’d need to use temporary storage for token counts, which negatively impacts performance.

Additional context
The Python ADK includes a usage_metadata field in its LlmResponse implementation.