[REQ][KOTLIN-SPRING] FEAT: Spring Declarative HTTP Interface for easy client instantiation

[Picazsoo]

Is your feature request related to a problem? Please describe.

I would like to provide a library option for kotlin-spring generator that will enable generating of a Spring Declarative Http Interface for simple instantiations of clients that is available in Spring Boot 3+

Describe the solution you'd like

My goal is to provide only the barebones interface classes. Stuff like creating the bean itself via HttpServiceFactory will be left to the user as generating that provides minimal value. I plan on providing option to generate method return types as ResponseEntity<SomeClassType> or just SomeClassType and to support both reactive and non-reactive option.

Describe alternatives you've considered

An alternative currently available in kotlin-spring generator is the spring-cloud library that generates feign clients. However the declarative http interface approach is even simpler and it works with both reactive and non reactive implementations and the developer can freely choose whether to instantiate the interface with either WebClient, RestTemplate, or RestClient.

Additional context

I already have this working in my local project and I think this would be a sensible contribution to this wonderful project.

[Picazsoo]

For reactive my plan is to support both coroutines and reactor style.

So for coroutines mode the method signature would be e.g.

• suspend fun someMethod() : Something (non-wrapped)
• suspend fun someMethod() : ResponseEntity<Something> (wrapped)
• suspend fun someMethod() : List<Something> (non-wrapped) note: Flow is not supported in spring declarative interface
• suspend fun someMethod() : ResponseEntity<List<Something>> (wrapped) note: Flow is not supported in spring declarative interface

and for reactor mode it would be e.g.

• fun someMethod() : Mono<Something> (non-wrapped)
• fun someMethod() : Mono<ResponseEntity<Something>> (wrapped)
• fun someMethod() : Flux<Something> (non-wrapped)
• fun someMethod() : Mono<ResponseEntity<Flux<Something>>> (wrapped) note: This looks terrible but my understanding is that this is the correct way to represent it.

[Picazsoo]

I implemented the necessary changes and the PR is ready for review.