[REQ] [kotlin-spring] Add clientRegistrationId for spring-declarative-http-interface (parity with Java spring)

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

When generating a Kotlin HTTP Interface client with kotlin-spring + library=spring-declarative-http-interface (Spring Boot 4 / Security 7), the generated API interfaces do not include @ClientRegistrationId. Without that annotation, Spring Security’s OAuth2 HTTP Service Client integration does not attach tokens for client_credentials (or other registrations).

The Java spring generator already supports this via clientRegistrationId (library=spring-http-interface, useSpringBoot4=true) — see PR #22726 and the spring generator docs. kotlin-spring has no equivalent option, so Kotlin users must post-process generated *Api.kt files.

Describe the solution you'd like

Add a clientRegistrationId config option to kotlin-spring with the same semantics as the Java spring generator:

• When set, and library=spring-declarative-http-interface (with Boot 4 / Security 7 as required), emit @ClientRegistrationId("<id>") on the generated interface.
• Document the option in kotlin-spring docs.

Example:

generatorName: kotlin-spring
library: spring-declarative-http-interface
additionalProperties:
  useSpringBoot4: "true"
  interfaceOnly: "true"
  clientRegistrationId: assistant

Expected:

@ClientRegistrationId("assistant")
interface DocumentsApi {
    // ...
}

Describe alternatives you've considered

1. Gradle doLast workaround: inject @ClientRegistrationId into generated *Api.kt after generation (what we do today).
2. Switch the client generator to Java spring + spring-http-interface (works, but mixes Java clients into a Kotlin service and splits server/client generators).
3. Manual/non-generated HTTP interfaces.

Additional context

• Spring Security docs: HTTP Service Clients Integration (@ClientRegistrationId on the interface or methods).
• Java support: spring generator clientRegistrationId, PR #22726.
• Kotlin declarative HTTP interfaces: PR #22302 / spring-declarative-http-interface; no clientRegistrationId in kotlin-spring docs.
• openapi-generator version: 7.25.0.

Start by comparing the Java spring generator's clientRegistrationId implementation with the kotlin-spring generator and its spring-declarative-http-interface templates. Update the Kotlin generator documentation and verify that the option emits @ClientRegistrationId on generated interfaces when the stated library and Spring Boot settings are used.