format-table: allow to explicitly override JSON field names

In some cases it's useful to explicitly generate the JSON field names to
generate for table columns, instead of auto-mangling them from table
header names that are intended for human consumption.

This adds the infra and a test for it.

It's intended to be used by #20544, for the first column, which in text
mode should have an empty header field, but have an explicit name in
json output mode.