Add `Examples` class

[pplonski]

Add a mr.Examples component for loading predefined widget values.

API

mr.Examples(
    examples={
        "Young customer": {
            "age": 25,
            "country": "Poland",
        },
        "Enterprise customer": {
            "age": 45,
            "country": "Germany",
        },
    }
)

Dictionary keys should match existing Mercury widget key or url_key.

Behavior

Show available examples in the app:

Examples

[ Young customer ]
[ Enterprise customer ]

When an example is clicked:

1. Find widgets by key, then url_key.
2. Set all provided widget values.
3. Trigger notebook execution once after all values are updated.

Missing widget keys should produce a warning.

V1 scope

• predefined examples only
• explicit widget values
• no automatic widget discovery
• no saving/editing examples from the UI
• no outputs or scenario management

Goal: let users quickly populate an app with meaningful example inputs.