Mini App Composer Extensions (replacing composer actions)

[deodad]

This proposal is to replace Composer Actions with Mini App Composer Extensions that offer similar functionality but unified under the Mini Apps standard. Mini Apps could support a Composer Extension by setting a composerExtensionUrl in their manifest.

When a user is composing a cast they would be able to view a list of their Mini Apps that function as a composer extension. Selecting a Mini App from this menu would open the Mini App at composerExtensionUrl with the state of the composer provided in a query parameter and also available in context.location.

The user is able to interact with the Mini App like normal with expectation of being returned to the composer via composeCast at some point.

Migration path for existing composer actions
Provide a way for mini app developers to start building and testing composer extensions. Set a cut off date in the future where a hard cutover is done. Given the current list is global and the new list is based on your added mini apps there will be some disruption but given the relatively low usage of these this is acceptable.

Passing composer state in the URL
To avoid making developers marshal data from a handful of query parameters (including arrays) we can use a single query parameter and pass the composer state in a base64Url encoded JSON serialization of an object that matches the same format provided in context.location and used by composeCast:

{
  text?: string;
  embeds?: [] | [string] | [string, string];
  parent?: {
      type: 'cast';
      hash: string;
  };
  channelKey?: string;
 }