libdm: report: support reporting field IDs in headings

libdm: report: support reporting field IDs in headings

Add new DM_REPORT_OUTPUT_FIELD_IDS_IN_HEADINGS report output flag.
If enabled, column IDs are reported instead of column names in report
headings.

The 'column IDs' are IDs as found in 'const struct dm_report_field_type *fields'
array that is passed during report initialization (that is, a call to
dm_report_init/dm_report_init_with_selection). In this case, the 'id'
dm_report_field_type member is used instead of the 'heading' member.