Add a new method for registering Webhooks callback endpoints #236

Reason/Context

Microcks 1.14.0 came withe OpenAPI Callbacks and Webhooks support (see our guide here: https://microcks.io/documentation/guides/usage/openapi-webhooks/). As the application we're developing can be a consumler of such excternal webhooks, it's super useful to have the ability to test that it actually behave correctly when receiving these webhook callbacks.

Description

The purpose of this issue is to add a new .withWebhookRegistration() method that allows registrating a webhool callback in Microcks container so that your application will start receving these notifications and you then will be able to assert it behaves correctly.

Implementation ideas

Take care of encoding conventions and check microcks/microcks-testcontainers-java#335