Added functionality for callbacks

[h4ck4l1]

• Added functionality of Click event callback
• Added wasm-yew-minimal-callback example in example docs

[h4ck4l1]

@andrei-ng please notify any changes, I am unsure when I will respond next week but I will make sure changes take place atmost in next weeks weekend.

1. I thought of adding everything in binding but callbacks even the next Select callback need their own structs and higher level SelectEvent structs
2. All the fields are Optional as using serdes default attribute gives serialization error for reasons beyond me.
3. Added the minimal Yew example
4. If this goes through implementing select,hover and rest are on the way

Best Regards

[andrei-ng]

Thank you for the PR! I will review it during the week.