cffi: always return config values as JSON

Previously, string JSON values were special cased to be provided as
bare strings, which means that CFFI modules have to either know what
type each value is expected to be, or use a heuristic such as trying to
decode and then treating the value as a string on failure.

Instead, we can always return JSON, and let the downstream consumer
handle deserialising the value into whatever type is expected.

The new behaviour is gated on a new ABI version 2: modules built against
version 1 will continue to get the old behaviour.

resources/custom_modules/cffi_example/main.c

@@ -17,7 +17,7 @@ void onclicked(GtkButton* button) {
 }
 
 // You must
-const size_t wbcffi_version = 1;
+const size_t wbcffi_version = 2;
 
 void* wbcffi_init(const wbcffi_init_info* init_info, const wbcffi_config_entry* config_entries,
                   size_t config_entries_len) {
@@ -67,4 +67,4 @@ void wbcffi_refresh(void* instance, int signal) {
 
 void wbcffi_doaction(void* instance, const char* name) {
   printf("cffi_example inst=%p: doAction(%s)\n", instance, name);
-}
+}