better attachment support

[ldub]

Having read a lot of Slack documentation and operated v2.0.0.3 in production for a bit, I now understand that the Slack message attachment field is poorly standardized. None of the fields are required and there are different versions of the attachments api over the years that are all supported in this one field.

To make apps that look at attachments, chance are, you're going to need to look at the raw json value. So I've edited slack-web to try to decode the attachment based on documented schema (and ported from Slack's own open source javascript sdks) but then also pass the raw json value to clients.

Before submitting your PR, check that you've:

• Documented new APIs with Haddock markup
• Added @since declarations to the Haddock

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Bumped the version number if there isn't an (unreleased) on the Changelog
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)