db.columns: Add JSON support

[NishantBansal2003]

Fixes: #6008

This PR adds JSON support to the db.columns module. The JSON output looks like:

[
    "cat",
    "OBJECTID",
    "WAKE_ZIPCO",
    "PERIMETER",
    "ZIPCODE_",
    "ZIPCODE_ID",
    "ZIPNAME",
    "ZIPNUM",
    "ZIPCODE",
    "NAME",
    "SHAPE_Leng",
    "SHAPE_Area",
]

This PR includes the following changes:

1. Adds a format option with plain, and json modes for output formatting.
2. Adds tests covering each of the new formats.
3. Adds a Python example to the documentation for parsing JSON output.