Fix #8822: Some procedures containing LIST aggregate function are not restored in Firebird 6.0

[dyemanov]

Since a new BLR verb is required anyway, I've added just one (for custom order) instead of duplicating both blr_agg_list_* verbs.

Also, the BLR printer is extended to support (optionally) WITHIN GROUP clause in other aggregate functions.

Once merged, PERCENTILE_CONT and PERCENTILE_DISC functions (PR #8807) should also be changed to utilize blr_within_group_order.

[dyemanov]

BLR output after the patch, for the COUNT / legacy LIST / ordered LIST:

1,0,
blr_agg_count_distinct,
    blr_field, 0, 18, 'R','D','B','$','P','R','O','C','E','D','U','R','E','_','N','A','M','E',
2,0,
blr_agg_list_distinct,
    blr_field, 0, 18, 'R','D','B','$','P','R','O','C','E','D','U','R','E','_','N','A','M','E',
    blr_literal, blr_text2, 0,0, 2,0, 59,32,
3,0,
blr_agg_list_distinct,
    blr_field, 0, 18, 'R','D','B','$','P','R','O','C','E','D','U','R','E','_','N','A','M','E',
    blr_literal, blr_text2, 0,0, 2,0, 59,32,
    blr_within_group_order, 1,
        blr_ascending,
            blr_field, 0, 18, 'R','D','B','$','P','R','O','C','E','D','U','R','E','_','N','A','M','E',