coredump: add JSON output support to coredumpctl info

coredump: add JSON output support to coredumpctl info

Implement support for the --json= flag in the info subcommand
(issue #38844). Previously, coredumpctl info always produced
human-readable text output regardless of --json=.

Add a CoredumpFields struct that holds all journal fields extracted
for a coredump entry, along with coredump_fields_done() to release
member resources and coredump_fields_load() to populate the struct
from a journal entry. Both print_info() and the new print_info_json()
use this shared loader, eliminating the duplicate RETRIEVE loop.

print_info_json() builds a JSON object with the same fields shown by
print_info(). Missing fields are omitted via SD_JSON_BUILD_PAIR_CONDITION,
matching the tolerant behavior of print_info() rather than skipping the
entry entirely. Signal/Reason handling mirrors print_info(): normal
coredumps (MESSAGE_ID == SD_MESSAGE_COREDUMP_STR) emit a numeric Signal
field; non-normal entries (kernel oops, etc.) emit a Reason field with
the raw text from COREDUMP_SIGNAL.

Co-developed-by: Claude Opus 4.6 <noreply@anthropic.com>