feat: role-specific pool configuration

[levkk]

Feature description

Implement role-specific configuration for connection pools.

Example:

[[users]]
name = "pgdog"
database = "prod"
pool_size_replica = 10
pool_size_primary = 25

[[databases]]
name = "prod"
host = "10.0.0.0"
pool_size_replica = 25

This allows for replica and primary connection pools in the same cluster to have different settings. Just like all other settings, user values take priority over database values.

Notes

Pools are now role-aware. The role is propagated down from the load balancer to the pool, so the pool is mutated by the load balancer. If the pool is created differently, that mutation doesn't take place, and the pool role will not be set correctly. This is expected, since pool roles only make sense in the context of the load balancer.

Cleanup

• Removed dead code in pool::Config
• Removed redundant accessors in pool::Config
• Moved load balancer atomic role into its own struct

Related issue

Fixes #1295.

[codecov]

Codecov Report

❌ Patch coverage is 96.91358% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/backend/pool/config.rs	88.09%	5 Missing ⚠️

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