[libdnf, actions plugin] Implement "json" communication mode

Processes send requests and the actions plugin responds.

- Request format:
{"op":"<operation>", "domain":"<domain>", "args":{<op_domain_spec_args>}}

"domain" is ignored in the case of the "log" operation.

- Response format:
{"op": "reply", "requested_op":"<op_from_request>", "domain":
"<domain_from_request>", "status": "OK", "return":{<data>}}

{"op": "reply", "requested_op":"<op_from_request>", "domain":
"<domain_from_request>", "status": "ERROR", "message":"<error message>"}

"return" is not included in the response to a request for a "log"
operation.

If the JSON request is unparsable - a syntax error or incomplete message,
or the response cannot be written, the action plugin closes both communication
pipes. The process must terminate if the communication pipes are closed.

libdnf5-plugins/actions/CMakeLists.txt

@@ -11,6 +11,10 @@ add_library(actions MODULE actions.cpp)
 # disable the 'lib' prefix in order to create actions.so
 set_target_properties(actions PROPERTIES PREFIX "")
 
+pkg_check_modules(JSONC REQUIRED json-c)
+include_directories(${JSONC_INCLUDE_DIRS})
+target_link_libraries(actions PRIVATE ${JSONC_LIBRARIES})
+
 target_link_libraries(actions PRIVATE libdnf5)
 
 install(TARGETS actions LIBRARY DESTINATION "${CMAKE_INSTALL_FULL_LIBDIR}/libdnf5/plugins/")