Ability to render Custom Attachment Type

[nosmirck]

The api provides a way to send a custom payload through attachments and we can have a MessageBuilder that easily handles this scenario:

MessageListView(
  showVideoFullScreen: true,
  messageBuilder:_buildCustomMessage,
  showOtherMessageUsername: true,
  threadBuilder: (_, parentMessage) {
  return ThreadPage(
  parent: parentMessage,
  );
  },
),

and in _buildCustom Message logic for it can be done, however, it would basically requiere to re-write the MessageWidget with all its features + a small set of builders for the attachments.

Proposed Solution:

Add a list of AttachmentBuilders to the MessageListView that can be passed down to the MessageWidget and it can be used during _buildBubble

[imtoori]

In the alpha version the MessageListView has a property Map<String, AttachmentBuilder> attachmentBuilders.
The key is the type of the attachment and the value is the builder.
It defaults to the regular attachment builders