Add support for enterprise/org-wide installations

Support was added in bolt v3 #106 but wasn't enabled or implemented

Do we want to support this? 🤷‍♂️
Could be straightforward to implement, not sure how to test ...

https://api.slack.com/enterprise/apps
https://api.slack.com/enterprise/apps/migration
https://slack.dev/bolt-js/tutorial/migration-v3