Add valkey/redis driver with operator-secret auto-configuration

Apps fronted by valkey-operator instances need the same wait-for-ready
initContainer postgres workloads already get, and wiring per-field
secretKeyRefs for operator-generated secrets is boilerplate. waitfordb
now speaks enough RESP to AUTH and PING, and natively understands the
secret shapes CNPG and valkey-operator generate so an initContainer is
just envFrom plus a mode variable.

- valkey driver (alias: redis): fresh TCP connection per attempt,
  optional AUTH (ACL user or default), PING, reusing the existing
  retry/backoff wait loop; redis:///valkey:// DSNs; default port 6379
- WAITFORDB_SECRET_FORMAT=cnpg|valkey for envFrom-injected operator
  secrets: CNPG <cluster>-app host/port/dbname/user/password keys, and
  valkey-operator key-per-username secrets (_operator preferred, or
  WAITFORDB_USER's same-named key)
- autodetection from injected keys (CNPG keys -> postgres, valkey keys
  -> valkey); explicit WAITFORDB_DRIVER/WAITFORDB_* always win, PG*
  fallback and all existing flags unchanged
- valkey needs no user/database to be valid (unauthenticated PING)
- tests: secret shape parsing/autodetect/mismatch, fake RESP server
  covering NOAUTH/WRONGPASS/ACL auth/DSN, retry-until-up wait
- README: envFrom initContainer snippets for CNPG and valkey-operator

internal/driver/driver_test.go

@@ -8,7 +8,7 @@ import (
 )
 
 func TestGetRegisteredDrivers(t *testing.T) {
-	for _, name := range []string{"postgres", "mysql"} {
+	for _, name := range []string{"postgres", "mysql", "valkey"} {
 		if _, err := Get(name); err != nil {
 			t.Errorf("Get(%q) failed: %v", name, err)
 		}