Add logging during awskms auto-unseal (#9794)

Adds debug and warn logging around AWS credential chain generation,
specifically to help users debugging auto-unseal problems on AWS, by
logging which role is being used in the case of a webidentity token.

Adds a deferred call to flush the log output as well, to ensure logs
are output in the event of an initialization failure.

physical/dynamodb/dynamodb.go

@@ -187,6 +187,7 @@ func NewDynamoDBBackend(conf map[string]string, logger log.Logger) (physical.Bac
 		AccessKey:    conf["access_key"],
 		SecretKey:    conf["secret_key"],
 		SessionToken: conf["session_token"],
+		Logger:       logger,
 	}
 	creds, err := credsConfig.GenerateCredentialChain()
 	if err != nil {