feat: add slackv2 notification

[eschutho]

SUMMARY

SeeSIP

This PR adds a new Notification SlackV2Notification that uses the Slack upload files v1 method. It is required for slack workspace owners to add the channels:read scope to their workspace in order to use this api. It also fetches all slack channels in order to convert user-inputed names to api-required ids. Because of this, I added a UI to support this.

Per the SIP, the both the api and UI will attempt to convert the notification to SlackV2 if the scopes are in place.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

After:

https://github.com/apache/superset/assets/5186919/01c6b398-faf8-49b0-99fb-ce42765d7d70

TESTING INSTRUCTIONS

Create a new slack application and connect it to Superset.

ADDITIONAL INFORMATION

• [ ] Has associated issue:
• [ ] Required feature flags:
• [ ] Changes UI
• [ ] Includes DB Migration (follow approval process in SIP-59)
  • [ ] Migration is atomic, supports rollback & is backwards-compatible
  • [ ] Confirm DB migration upgrade and downgrade tested
  • [ ] Runtime estimates and downtime expectations provided
• [ ] Introduces new feature or API
• [ ] Removes existing feature or API