Support __spqr__auto_distribution hint

[reshke]

Also __spqr__distribution_key

db1=# \c spqr-console

		SPQR router admin console
	Here you can configure your routing rules
------------------------------------------------
	You can find documentation here
https://github.com/pg-sharding/spqr/tree/master/docs

psql (16.2 (Ubuntu 16.2-1.pgdg20.04+1), server console)
WARNING: psql major version 16, server major version 0.0.
         Some psql features might not work.
You are now connected to database "spqr-console" as user "reshke".
spqr-console=> show relations;
 Relation name | Distribution ID | Distribution key
---------------+-----------------+------------------
 t             | ds1             | ("i", identity)
 t22           | ds1             | ("i", identity)
(2 rows)

spqr-console=> \c db1
psql (16.2 (Ubuntu 16.2-1.pgdg20.04+1), server 15.4)
You are now connected to database "db1" as user "reshke".
db1=# create table t33(i int, j int)  /* __spqr__auto_distribution: ds1, __spqr__distribution_key: i  */;
CREATE TABLE
db1=# \c db1
psql (16.2 (Ubuntu 16.2-1.pgdg20.04+1), server 15.4)
You are now connected to database "db1" as user "reshke".
db1=# \c spqr-console

		SPQR router admin console
	Here you can configure your routing rules
------------------------------------------------
	You can find documentation here
https://github.com/pg-sharding/spqr/tree/master/docs

psql (16.2 (Ubuntu 16.2-1.pgdg20.04+1), server console)
WARNING: psql major version 16, server major version 0.0.
         Some psql features might not work.
You are now connected to database "spqr-console" as user "reshke".
spqr-console=> show relations;
 Relation name | Distribution ID | Distribution key
---------------+-----------------+------------------
 t             | ds1             | ("i", identity)
 t22           | ds1             | ("i", identity)
 t33           | ds1             | ("i", identity)
(3 rows)

spqr-console=>