Enable Test API Generator for WSDL

[tschlat]

User story
The Test API generator (#1156) provides code to create an OpenAPI from a WSDL. This OpenAPI can then be used to generate a Test API.

As a user I want to be able to:

1. Perform the transformation via Maven
2. Let the citrus-test-api-generator-maven-plugin perform the transformation automatically

For 1., a specific goal in the citrus-test-api-generator-maven-plugin is needed.
For 2., the respective plugin has to be adjusted to automatically transform a wsdl source into an OpenAPI and use that for generation.

When finished, check for all occurrences of TODO #1163 in Citrus code base..