add get_watermark helper function

[rapimo]

we already query some watermark infos on scanning here

kafka_fdw/src/kafka_fdw.c

Line 1127 in 9dc5eaa

err = rd_kafka_query_watermark_offsets(

What would be great is having some helper function to present this data to the end user

e.g.

SELECT  * FROM kafka_get_watermarks('table_name');
 partition | offset_low | offsset_high
-----------+------------+--------------
         1 |       1001 |         3001
         2 |       1002 |         3002
         3 |       1003 |         3003
         4 |       1004 |         3004
         5 |       1005 |         3005
         6 |       1006 |         3006
         7 |       1007 |         3007
         8 |       1008 |         3008
         9 |       1009 |         3009
        10 |       1010 |         3010
        11 |       1011 |         3011
        12 |       1012 |         3012
(12 rows)

[zilder]

Created a PR for this issue, see #34.

[rapimo]

closed by #34