feat: classical HttpClient GET methods + generation fixes (1.1.0)

[krusche]

Summary

Makes the classical (non-httpResource) generation mode usable and fixes several code-generation bugs discovered while adopting this generator for the Artemis OpenAPI client. Switches publishing to Maven Central and bumps the version to 1.1.0.

Changes

• Classical HttpClient.get() GET methods (useHttpResource=false): GET operations are now emitted as classical Observable methods in the API service. Previously every operation in api-service.mustache was wrapped in {{^vendorExtensions.x-is-get}}, so GETs were only ever produced via the httpResource resource template — disabling useHttpResource silently dropped all GET endpoints. GET-only tags now also keep their -api.ts file in classical mode.
• Relative model imports: model-to-model imports are now './other-model' instead of the bare 'other-model', which did not resolve.
• Observable<void>: operations always emit an explicit response type parameter, so void endpoints return Observable<void> instead of Observable<Object>.
• Mutation body argument: POST/PUT/PATCH always pass a body argument (HttpClient requires it; a missing body produced a TS "Expected 2–3 arguments" error).
• Path-parameter interpolation: non-numeric path params are interpolated as ${param} instead of the literal text {param}; numeric params no longer get a doubled $.
• Array-of-enum properties: wrapped in Array<…> instead of a single enum type.
• Build against openapi-generator 7.21.0 (aligned with the org.openapi.generator Gradle plugin version used downstream in Artemis).
• Publish to Maven Central: replaces GitHub Packages publishing (which requires authentication even to download) with the Sonatype Central Portal via the com.vanniktech.maven.publish plugin, so consumers resolve the generator from a plain mavenCentral() with no auth. Artifacts are GPG-signed, gated on a key being present so publishToMavenLocal still works key-free. Adds RELEASING.md.

Context

These fixes are required for the Artemis client to compile and run after switching its OpenAPI client to this generator (Artemis PR ls1intum/Artemis#13038). Configured there with useHttpResource=false, useInjectFunction=true, separateResources=false, readonlyModels=false.

Releasing 1.1.0 (maintainers)

Publishing now targets Maven Central (auth-free for consumers). One-time setup — see RELEASING.md:

1. Claim the de.tum.cit.aet namespace on central.sonatype.com (verify tum.de via a DNS TXT record).
2. Generate a Central Portal user token and a GPG signing key (publish the public key to a keyserver).
3. Add four repo secrets: MAVEN_CENTRAL_USERNAME, MAVEN_CENTRAL_PASSWORD, SIGNING_KEY (ASCII-armored private key), SIGNING_PASSWORD.

Then release by pushing a tag: git tag v1.1.0 && git push origin v1.1.0 — the workflow signs and uploads to the Central Portal and creates a GitHub Release.

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