Add progress bar API for LLM response

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

**Is your feature request related to a problem? Please describe.**
Right now, when the context is very big, LLM takes lot of time to respond. A progress bar to understand the progress will be helpful.

**Describe the solution you'd like**
A clear and concise description of what you want to happen.

**Describe alternatives you've considered**
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**Additional context**
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