Indicate explicitly blocked sources as "REDACTED" instead of "UNKNOWN" in `JsonLocation`

[cowtowncoder]

(note: follow-up to #991 changes)

Currently (up to Jackson 2.15) there is no distinction between two input indicators (ContentReference) that can be included in JsonLocation instead of proper ContentReference:

1. Missing source (either not available or not passed) and
2. Blocked source (StreamReadFeature.INCLUDE_SOURCE_IN_LOCATION disabled)

and both are described as "UNKNOWN". But it would be useful to distinguish these so that second case would indicate something like:

REDACTED (`StreamReadFeature.INCLUDE_SOURCE_IN_LOCATION` disabled)

leaving plain UNKNOWN for case where content reference is truly missing.

The main reason for this is that since (... if) we change default inclusion to more secure it is otherwise hard for users/developers to realize what is needed to enable source inclusion in case they do want to expose it.