Improve system to support rest resources with separated create and update endpoints

[gessnerfl]

Story

As a developer
I want that the Rest Resource are adopted such as that it supports Create and Update operations instead of only providing an Upsert operation
So that the Rest Resource fits to endpoints of the instana API implementing such a behaviour.

Implementation Details

Currently the all resources which are implemented by the provider are using a common pattern with a PUT endpoint for create and update of the resource. However resources such as web site monitoring or users have separated endpoints for create and update. The implementation should be adjusted such as that this pattern is supported. Therefore the interface should be adopted and a base implementation should be provided where POST is used for create and PUT is used for update operations.

[gessnerfl]

Generic implementation will be provided with the first implementation which requires create and update to be separated