amp-form: support auto-growing text input field

[sebastianbenz]

...similiar to how text input works when composing a text message in android (not sure of this approach is still up-to-date or if using contenteditable would be better).

Use Cases:

• comment input field
• review input field

[camelburrito]

@sebastianbenz please file on ampstart

[sebastianbenz]

@camelburrito can do - out of curiosity: how can ampstart solve this? I'd assumed that this is something that needs to be supported by the runtime?

[aghassemi]

@cvializ launched this recently 🎉 #20772