feat(templates): add Opt In Builder template

[cweems]

Description

Creates an embeddable, compliant opt-in form that can be used for short code applications and A2P 10DLC. Key features:

• Live editor allows you to preview your signup form.
• Customizable logo, content, and colors.
• Generate compliance language and terms of service.
• Track opt-ins via webhook, Segment, or Airtable.
• Supports one-click keyword opt-in on Android and iOS.

See README for setup details.

Checklist

• [x] I ran npm test locally and it passed without errors.
• [x] I acknowledge that all my contributions will be made under the project's license.

Related issues

[cweems]

@myfancypants Hey there! Is there a process for me to request a review on this so that I can merge and add it to the CodeExchange?

There's a bunch of functionality in this template, so I recorded a video demo of what it does here: https://www.loom.com/share/c931fc90f745403795153bb171b1e1a1

The intent is to help out all of our messaging customers that now need to capture opt-in for long codes under A2P 10DLC.