Build sherpa-onnx as a single shared library

[csukuangfj]

When -D BUILD_SHARED_LIBS=ON is passed to cmake, it builds a single shared library.

Specifically,

• For C APIs, it builds libsherpa-onnx-c-api.so
• For Python APIs, it builds _sherpa_onnx.cpython-xx-xx.so
• For Kotlin and Java APIs, it builds libsherpa-onnx-jni.so

There is no libsherpa-onnx-core.so any longer.

Note it affects only shared libraries.