Allow structured 'json.context.prefix' to apply to MDC and not key/value pairs

[philwebb]

See #45218 (comment)

Offering a general purpose property is a little tricky since Logback and Log4J2 have different feature sets.

[madanhk18]

\assign-me

[philwebb]

Thanks for the offer @madanhk18, but this one will require a little design work so we aren't ready to accept contributions just yet.