feat: OAuth2 client initial work

[betodealmeida]

SUMMARY

We're planning to introduce OAuth2 client management to Superset, so the clients can be configured on the app, and assigned to multiple databases.

This PR is a first step in that direction, allowing client configuration to be stored in a the database encrypted_extra parameter. This allows us to add support for OAuth2 to more DB engine specs before starting the work on a full UI solution.

In addition to reading the client configuration from encrypted_extra this PR also refactors the logic for triggering OAuth2, introducing a new method called needs_oauth2 to the DB engine spec. This is needed to give more flexibility to account for different OAuth2 providers.

The PR also introduces a new trigger for OAuth2, when the raw_connection fails due to needing a personal user token.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A

TESTING INSTRUCTIONS

Added tests.

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

[eschutho]

LGTM