feat: weighted load balancing

[levkk]

fix #575

How it works

Using weighted round robin. Each database host can specify the lb_weight, like so:

[general]
load_balancing_strategy = "weighted_round_robin"

[[databases]]
name = "prod"
host = "10.0.0.1"
lb_weight = 50

[[databases]]
name = "prod"
host = "10.0.0.1"
role = "replica"
lb_weight = 75

The lb_weight can be between 0 and 255. The default value is 255, so if not specified, it'll work like regular round robin. Only the relative weights matter, not total. For example, if you want a 80/20 split, you can do this:

[[databases]]
# [...]
lb_weight = 4

[[databases]]
# [...]
lb_weight = 1

[codecov]

Codecov Report

❌ Patch coverage is 98.47328% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog-config/src/database.rs	75.00%	1 Missing ⚠️
pgdog/src/backend/pool/lb/mod.rs	95.45%	1 Missing ⚠️

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