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ARROW-6061: [C++] Add ARROW_JSON feature flag for configuring arrow builds without RapidJSON

Tested this in our build environment (which doesn't have rapidjson) and confirmed this patch works.

Closes #4962 from hatemhelal/arrow-6061 and squashes the following commits:

af7fada9e <Hatem Helal> Conditionally add_subdirectory(json)
da14cbb57 <Hatem Helal> enable IPC when building arrow tests
17fce13c6 <Hatem Helal> simplify setting ARROW_WITH_RAPIDJSON and force turning on IPC when Flight is a requested target
3b67debed <Hatem Helal> cmake-format changes
ac9cfc755 <Hatem Helal> Error if integration tests are built without JSON and restore previous structure of ThirdpartyToolchain
27201ca12 <Hatem Helal> Add ARROW_JSON cmake option
148195d86 <Hatem Helal> Check for rapidjson dependency before building arrow/json

Authored-by: Hatem Helal <hhelal@mathworks.com>
Signed-off-by: François Saint-Jacques <fsaintjacques@gmail.com>