Add example prompt field to tools

[nsarrazin]

Describe your feature request

This lets the user specify a prompt that would call the tool. It can be shown as a demo if you're not sure how to use a tool.

We should show it somewhere in the UI so the user can easily start a conversation from that demo.

It can also be used for validating that a tool works. (run the example server-side, if the tool does not get called or does not return an output then something is wrong and dont let users publish it)

Implementation idea

Storing the prompt itself is straightforward since you can just store it as a string. Most tools use file inputs though so we should ideally also support that, which means storing example files in the DB.