fix: Static library compilation in podspec

[steve228uk]

Summary

• Updates the podspec configuration used when USE_FRAMEWORKS=static.
• Explicitly preserves React Native's clang C++ language standard in the merged pod target config.
• Explicitly sets the generated Swift Objective-C header name.
• Appends the required React Native and generated Swift header search paths without replacing existing pod target config.

Issues Resolved

Static framework iOS builds could fail because Objective-C sources attempted to import react_native_exponea_sdk-Swift.h before Xcode exposed it reliably.

The previous direct pod_target_xcconfig[...] approach could also fail during pod install because CocoaPods exposes pod_target_xcconfig= as a writer, but not as a readable hash accessor.

When overriding pod target config for static framework builds, the podspec must continue to carry React Native's configured clang C++ language standard so the target stays aligned with the React Native build settings.

How

• Reads any existing pod_target_xcconfig from s.attributes_hash.
• Preserves existing config values and existing HEADER_SEARCH_PATHS.
• Falls back to $(inherited) when no header search paths are already present.
• Assigns the merged config back via s.pod_target_xcconfig = ....
• Sets CLANG_CXX_LANGUAGE_STANDARD to rct_cxx_language_standard().
• Keeps SWIFT_COMPILATION_MODE set to incremental for static framework builds.
• Sets SWIFT_OBJC_INTERFACE_HEADER_NAME to react_native_exponea_sdk-Swift.h.

Validation

• git diff --check
• ruby -c react-native-exponea-sdk.podspec