add automatic json conversion for attributes #541

Job	Run time
build	2m 32s
Julia 1.6 - ubuntu-latest - x64	3m 43s
Julia 1.6 - macOS-latest - x64	5m 19s
Julia 1.6 - windows-latest - x64	14m 21s
Julia 1.7 - ubuntu-latest - x64	3m 30s
Julia 1.7 - macOS-latest - x64	3m 58s
Julia 1.7 - windows-latest - x64	4m 18s
Julia nightly - ubuntu-latest - x64	5m 43s
Julia nightly - macOS-latest - x64	5m 43s
Julia nightly - windows-latest - x64	5m 34s
	54m 41s

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