add automatic json conversion for attributes #540

Job	Run time
build	3m 33s
Julia 1.6 - ubuntu-latest - x64	4m 55s
Julia 1.6 - macOS-latest - x64	4m 19s
Julia 1.6 - windows-latest - x64	12m 48s
Julia 1.7 - ubuntu-latest - x64	3m 31s
Julia 1.7 - macOS-latest - x64	4m 41s
Julia 1.7 - windows-latest - x64	5m 17s
Julia nightly - ubuntu-latest - x64	5m 3s
Julia nightly - macOS-latest - x64	5m 29s
Julia nightly - windows-latest - x64	6m 53s
	56m 29s

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