Build sherpa-onnx as a single shared library (#1078)

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.

cmake/kaldifst.cmake

@@ -43,8 +43,23 @@ function(download_kaldifst)
 
   list(APPEND CMAKE_MODULE_PATH ${kaldifst_SOURCE_DIR}/cmake)
 
+  if(BUILD_SHARED_LIBS)
+    set(_build_shared_libs_bak ${BUILD_SHARED_LIBS})
+    set(BUILD_SHARED_LIBS OFF)
+  endif()
+
   add_subdirectory(${kaldifst_SOURCE_DIR} ${kaldifst_BINARY_DIR} EXCLUDE_FROM_ALL)
 
+  if(_build_shared_libs_bak)
+    set_target_properties(kaldifst_core
+      PROPERTIES
+        POSITION_INDEPENDENT_CODE ON
+        C_VISIBILITY_PRESET hidden
+        CXX_VISIBILITY_PRESET hidden
+    )
+    set(BUILD_SHARED_LIBS ON)
+  endif()
+
   target_include_directories(kaldifst_core
     PUBLIC
       ${kaldifst_SOURCE_DIR}/