Add citus_stat_user_tables UDF

[naisila]

DESCRIPTION: Add citus_stat_user_tables UDF

This UDF provides n_live_tup, n_dead_tup, n_tup_ins etc for each Citus table in the database, based on the definitions of these columns in the Postgres view pg_stat_user_tables.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.17%. Comparing base (9bc7060) to head (6e5e723).

Additional details and impacted files

@@                     Coverage Diff                     @@
##           naisila/citus_column_stats    #8054   +/-   ##
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  Coverage                       89.17%   89.17%           
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  Files                             284      284           
  Lines                           61515    61513    -2     
  Branches                         7697     7697           
===========================================================
  Hits                            54853    54853           
+ Misses                           4459     4455    -4     
- Partials                         2203     2205    +2     

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