citrus actions for REST api

as a user of both citrusframework/citrus and citrusframework/citrus-simulator I want to have standard "actions" that call a citrus API with the right arguments.

a good way of doing this is by extracting the OpenAPI specification from the simulator, then add a new module (name to be thought of) with generated clients. I would use some [java-generator](https://openapi-generator.tech/docs/generators/java/) for this, preferably one without spring so the end user is not dependent upon `citrus-spring`.

the specifications should be stored in one file per resource. they belong into the `simulator-starter` (soon to be `citrus-spring-boot-simulator`, see #193) module, because it is the root of the API. multiple files are better maintainable than one large file.

the server implementation should be based on the OpenAPI as well (interfaces only, the already present resources should then implement these. that ensures compatibility of both client and server at all times. use the [OpenAPI Spring generator](https://openapi-generator.tech/docs/generators/spring/).

it can be split into the following tasks whereas the first task must be done before all others:
- [ ] extract OpenAPI from v3/api-docs into files
- [ ] create a new module with clients
- [ ] switch the server implementation to the OpenAPI spring generator

Start with the simulator-starter module and its v3/api-docs output, then review the linked OpenAPI Java and Spring generator guidance. Extract one specification per resource before creating the client module and replacing the server implementation with generated interfaces. Done means all three checklist tasks are completed and client/server compatibility is based on the specifications.

Written by the indexing model from the issue text.