Add useSwiftJavaJniCore option to control generated imports

[ybiletskyi]

Note

Medium Risk
Generated Swift import sets change compile-time behavior for consumers toggling the new flag; default (false) preserves legacy imports. Signing change is low risk for CI releases.

Overview
Adds useSwiftJavaJniCore on the module package JSON (SwiftModuleDescriptor) so codegen can pick between the legacy JNI stack (import Java plus java_swift) and a swiftlang jni-core path that drops Java while keeping java_swift, JavaCoder, and AnyCodable. SwiftWriter.emitImports and every generator (SwiftValue, SwiftReference, SwiftDelegate, SwiftBlock, and bootstrap SwiftJava.swift) pass the flag through.

JavaSwiftProcessor now fails fast if the module descriptor is missing before writing SwiftJava.swift, instead of only after starting annotation processing.

Release tooling: PUBLISH_VERSION 0.9.7, Readdle Swift Gradle plugin 6.2.1, and GPG signing only when signing.keyId is set so publishToMavenLocal works without keys.

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