add types for AttachmentAction interface
add types for AttachmentAction interface

These types are based off of what I found at:
https://api.slack.com/docs/interactive-message-field-guide

slackapi/node-slack-sdk,slackapi/node-slack-sdk,slackapi/node-slack-sdk,slackapi/node-slack-sdk,slackhq/node-slack-client,slackhq/node-slack-sdk