Add WithCredentials to AWS auth API

The default behavior does not allow to fully customize the credentials used to access Vault thru AWS authentication.

The developer can choose to skip process environment variables and override the default behavior.

api/auth/aws/aws.go

@@ -298,3 +298,12 @@ func WithRegion(region string) LoginOption {
 		return nil
 	}
 }
+
+// WithCredentials can be used to specify a custom set of credentials
+// from the source you choose.
+func WithCredentials(creds *credentials.Credentials) LoginOption {
+	return func(a *AWSAuth) error {
+		a.creds = creds
+		return nil
+	}
+}