feat: Add integration testing JSON reader (#322)

This PR adds a `TestingJSONReader`. It uses
https://github.com/nlohmann/json to do the parsing. This doesn't
implement parsing a record batch or array yet, and there are a few
missing types (the same missing types as the writer). I'd like to do
those as a follow-up.

The testing for this is mostly roundtrip testing, which is maybe not
ideal but also maybe OK because the writer is an independent
implementation. When the ability to compare schemas for equality gets
added that can be tacked on to the roundtrip test utility function as
well.

CMakeLists.txt

@@ -203,6 +203,16 @@ if(NANOARROW_BUILD_TESTS)
 
   fetchcontent_makeavailable(googletest)
 
+  # JSON library for integration testing
+  # Also used by some versions of Arrow, so check if this is already available
+  if(NOT TARGET nlohmann_json::nlohmann_json)
+    fetchcontent_declare(nlohmann_json
+                         URL https://github.com/nlohmann/json/archive/refs/tags/v3.11.2.zip
+                         URL_HASH SHA256=95651d7d1fcf2e5c3163c3d37df6d6b3e9e5027299e6bd050d157322ceda9ac9
+    )
+    fetchcontent_makeavailable(nlohmann_json)
+  endif()
+
   add_executable(utils_test src/nanoarrow/utils_test.cc)
   add_executable(buffer_test src/nanoarrow/buffer_test.cc)
   add_executable(array_test src/nanoarrow/array_test.cc)
@@ -235,7 +245,11 @@ if(NANOARROW_BUILD_TESTS)
                         coverage_config)
   target_link_libraries(array_stream_test nanoarrow gtest_main coverage_config)
   target_link_libraries(nanoarrow_hpp_test nanoarrow gtest_main coverage_config)
-  target_link_libraries(nanoarrow_testing_test nanoarrow gtest_main coverage_config)
+  target_link_libraries(nanoarrow_testing_test
+                        nanoarrow
+                        gtest_main
+                        nlohmann_json::nlohmann_json
+                        coverage_config)
 
   include(GoogleTest)
   # Some users have reported a timeout with the default value of 5