[ECO-4927] Change `Metadata` to use `JSONValue` #668

Job	Run time
lint	57s
spec-coverage	49s
check-documentation	1m 18s
generate-matrices	49s
SPM (Xcode 16)	1m 51s
SPM, `release` configuration (Xcode 16)	1m 7s
Xcode, `release` configuration, macOS (Xcode 16)	1m 58s
Xcode, macOS (Xcode 16)	2m 43s
Xcode, `release` configuration, iOS (Xcode 16)	1m 40s
Xcode, iOS (Xcode 16)	4m 16s
Xcode, `release` configuration, tvOS (Xcode 16)	2m 4s
Xcode, tvOS (Xcode 16)	3m 12s
Example app, macOS (Xcode 16)	2m 31s
Example app, iOS (Xcode 16)	1m 56s
Example app, tvOS (Xcode 16)	1m 51s
all-checks-completed	1s
	29m 3s

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