DUX-3101: Reactions API support

[lf-]

This is motivated by needing it for slacklinker. No practical way to test this but I can confirm it works in practice on slacklinker :)

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)

[linear]

DUX-3101 Integrate reactions support in slack-web