feat: add primary only plugin

[levkk]

pgdog=# EXPLAIN SELECT * FROM sessions;
                               QUERY PLAN                               
------------------------------------------------------------------------
 Seq Scan on sessions  (cost=0.00..32.60 rows=2260 width=8)
 
 PgDog Routing:
   Summary: shard=0 role=primary
   Shard 0: SELECT omnishard round robin
   Note: plugin pgdog_primary_only_tables adjusted routing role=primary
(6 rows)

#823

[codecov]

Codecov Report

❌ Patch coverage is 6.31579% with 89 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/plugin/mod.rs	0.00%	38 Missing ⚠️
pgdog-plugin/src/logging.rs	0.00%	33 Missing ⚠️
pgdog-plugin/src/plugin.rs	0.00%	18 Missing ⚠️

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