[api] remove UART from in cli/ncp API (#6243)

This commit removes application library CLI/NCP dependency on platform
layer UART APIs. Instead, application layer provides callbacks sending
CLI/NCP data.

With this change, platforms with native support for formatted output
can simply implement the CLI output callback with something like
`vprintf()`.

examples/apps/ncp/ftd.cmake

@@ -28,6 +28,7 @@
 
 add_executable(ot-ncp-ftd
     main.c
+    ncp.c
 )
 
 target_include_directories(ot-ncp-ftd PRIVATE ${COMMON_INCLUDES})
@@ -37,6 +38,7 @@ target_link_libraries(ot-ncp-ftd PRIVATE
     ${OT_PLATFORM_LIB}
     openthread-ftd
     ${OT_PLATFORM_LIB}
+    openthread-ncp-ftd
     ${OT_MBEDTLS}
     ot-config
 )