chore: [OpenAPI] Reduce Spring dependency

[newtork]

Context

https://github.com/SAP/cloud-sdk-java-backlog/issues/464

We want to reduce the Spring dependency.

Currently:

• Users require explicit org.springframework:spring-core::compile
• Users require explicit org.springframework:spring-web::compile

Expected (1):

• [x] Legacy users are not negatively affected by change
• [x] Users do not require explicit org.springframework:spring-core::compile
• [x] Users do not require explicit org.springframework:spring-web::compile

Expected (2):

• [ ] Legacy users are not negatively affected by change
• [ ] Users do not require transitive org.springframework:spring-core::compile
• [ ] Users do not require transitive org.springframework:spring-web::compile

Feature scope:

• [x] Update mustache file to reduce Spring types from public API (70%)

  • [x] Weaken openapi-core API contract to allow for non-Spring typed arguments
  • [x] Newly generated service API classes use non-Spring typed code.
  • [x] Sample project is updated to motivate changes.

• [ ] Harden and improve openapi-core API contract

  • [ ] offer legacy weak API (for Spring and non-Spring typed arguments)
    • [ ] Spring classes are coming from optional-scoped dependencies.
  • [ ] offer new, improved and recommended API (for non-Spring-typed arguments)

• [ ] Update mustache file to use recommended API

Definition of Done

• [ ] Functionality scope stated & covered
• [ ] Tests cover the scope above
• [x] ~Error handling created / updated & covered by the tests above~
• [x] ~Documentation updated~
• [ ] Release notes updated