Add dbx library: PostgreSQL cluster with master/replica routing, retry, health checking

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

options.go

@@ -0,0 +1,39 @@
+package dbx
+
+// Option is a functional option for NewCluster.
+type Option func(*Config)
+
+// WithLogger sets the logger for the cluster.
+func WithLogger(l Logger) Option {
+	return func(c *Config) {
+		c.Logger = l
+	}
+}
+
+// WithMetrics sets the metrics hook for the cluster.
+func WithMetrics(m *MetricsHook) Option {
+	return func(c *Config) {
+		c.Metrics = m
+	}
+}
+
+// WithRetry overrides the retry configuration.
+func WithRetry(r RetryConfig) Option {
+	return func(c *Config) {
+		c.Retry = r
+	}
+}
+
+// WithHealthCheck overrides the health check configuration.
+func WithHealthCheck(h HealthCheckConfig) Option {
+	return func(c *Config) {
+		c.HealthCheck = h
+	}
+}
+
+// ApplyOptions applies functional options to a Config.
+func ApplyOptions(cfg *Config, opts ...Option) {
+	for _, o := range opts {
+		o(cfg)
+	}
+}