Add custom types section with sql.Scanner/driver.Valuer

[coolai86]

This PR adds guidance for using custom types with sqlc type overrides.

Changes

• Add / pattern documentation
• Add JSONB override configuration example
• Add nullable types guidance for Go 1.22+ () and legacy types
• Update PostgreSQL type mappings table to reference custom types

Why This Matters

sqlc type overrides work with any Go type implementing and . This is essential for:

• JSONB columns: Automatic JSON marshaling to Go structs (no manual )
• Domain-specific types: Currency, dates with timezones, etc.
• Custom nullable types: Go 1.22+ for types without legacy equivalents

Example Impact

Before:

type Tenant struct { PaymentConfig []byte }
var cfg PaymentConfig
json.Unmarshal(tenant.PaymentConfig, &cfg) // manual work

After:

type Tenant struct { PaymentConfig *types.JSONB }
var cfg PaymentConfig
tenant.PaymentConfig.Get(&cfg) // one-liner

This pattern was used successfully in the authd project for payment config, OAuth claim mappings, and session data.