Add ability to store custom `metadata` to be used with `matchJsonata`

[jamietanna]

Metadata should be namespaced i.e. if module openssf-security-scorecard exposes metadata.score and metadata.maintained_score, this would be possible to look up with matchJsonata against openssf-security-scorecard.maintained_score or similar

See https://github.com/renovatebot/renovate/pull/new/feat/enrichment-1a-metadata for some examples of how this could look