Create mechanism for custom authentication in managed HTTP connections

[thjaeckle]

Some HTTP APIs might e.g. support "Bearer" authentication, but provide a non-OAuth or non-OpenIdConnect endpoint for obtaining JWT. E.g. a custom "POST /authenticate" endpoint which delivers a JWT.

In order to make use of such authentication methods, support such custom authentication methods in a configurable way (e.g. using Ditto functions, placeholders, etc.). Example endpoint for inspiration: https://portal.whereversim.com/api/doc/swagger.html#/Authentication/Authenticate