Initial implmentation for json file build output

This uses a simple backing Map (HashMap), for collecting the
data. It's entirely driven by ProgressReporter and if not
requested by the user by specifying:
`-H:BuildOutputJSONFile=<file>` on the command entirely
disabled.

It also adds a JSON schema to the native image manual as
an asset.

docs/reference-manual/native-image/BuildOutput.md

@@ -189,6 +189,7 @@ Run `native-image --expert-options-all | grep "BuildOutput"` to see all build ou
 -H:±BuildOutputBreakdowns    Show code and heap breakdowns as part of the build output. Default: + (enabled).
 -H:±BuildOutputColorful      Colorize build output. Default: + (enabled).
 -H:±BuildOutputGCWarnings    Print GC warnings as part of build output. Default: + (enabled).
+-H:BuildOutputJSONFile=""    Print build output statistics as JSON to the specified file.
 -H:±BuildOutputLinks         Show links in build output. Default: + (enabled).
 -H:±BuildOutputPrefix        Prefix build output with '<pid>:<image name>'. Default: - (disabled).
 -H:±BuildOutputProgress      Report progress in build output. Default: + (enabled).