add is default flag to deployment target (#3938)

internal/models/deployment_target.go

@@ -41,6 +41,9 @@ type DeploymentTarget struct {
 
 	// Metadata is a JSONB column that stores arbitrary metadata about the deployment target
 	Metadata JSONB `json:"metadata" sql:"type:jsonb" gorm:"type:jsonb;default:'{}'"`
+
+	// IsDefault indicates whether this is the default deployment target for the cluster
+	IsDefault bool `gorm:"default:false" json:"is_default"`
 }
 
 // ToDeploymentTargetType generates an external types.PorterApp to be shared over REST