HTTP Action (OAuth) - Support OAuth-connection settings provider 'Generic Oauth2'

As copied from Flow - Allow a user to add a HTTP Action to the flow (part 3 - Auth) so that it can be reviewed and prioritised if required by a customer:

• [Todo] new rest end point to allow setting generic OAuth2 provider

We currently (9 Nov. 2021) only support Azure Active Directory (including MS Graph) and by adding the generic OAuth2 provider it will allow customers to integrate with a lot more third parties.

Initial estimate, to be confirmed in Dev Meeting if / when required: 1 week