Ability to add prompt chips to custom adk agents will be very useful

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

## 🔴 Required Information
*Please ensure all items in this section are completed to allow for efficient
triaging. Requests without complete information may be rejected / deprioritized.
If an item is not applicable to you - please mark it as N/A*

### Is your feature request related to a specific problem?
Currently there is no option to add prompt chips to the custom ADK agents we build. This limits us in enhancing the user experience.

### Describe the Solution You'd Like
Just like we can add prompt chips to agents built using agent designer, we should have the option to add prompt chips to custom ADK agents as well.

### Impact on your work
How does this feature impact your work, and what are you trying to achieve?
We were looking for a way to direct users to an agent that automatically triggers action on opening the agent, for example, form review—the form can be automatically rendered on opening the agent. Would be nice to have an auto-start action option. But since that is not possible, at least having the ability to provide prompt chips for custom agents would help us make the experience more intuitive.

### Willingness to contribute
Are you interested in implementing this feature yourself or submitting a PR?
(Yes/No) - I am not sure, I can certainly help.

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## 🟡 Recommended Information

### Describe Alternatives You've Considered
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you've considered and why they didn't work for you.

### Proposed API / Implementation
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pseudo-code example.

### Additional Context
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