[Feat]: Different output format for cli search

[matglas]

Describe the solution you'd like:

Add JSON as output to the archivistactl search to allow easy scripting.

User value:

Especcially for new developers that start to use archivista it would be helpful to use the cli to query attestations in some way and collect them by using the cli retrieve. If you can use a json output of the search you can retrieve all of the attestations by envelope.

Expected behavior:

Add --format json to search. This will output the data in a parsable usable form.

Proposed solution:

Anything else you would like to add:

Testing changes required:

Documentation changes required:

TBD