BackendTrafficPolicy "Budget" fields should be in struct

[robscott]

What happened:
In today's SIG-Network review for v1.3, we got good feedback that budgetPercent and budgetInterval should really be in a struct to result in budget.percent and budget.interval. Particularly if we ever think that we'd add a third field with the same prefix.

What you expected to happen:
It would be worth moving the following fields into a struct:

gateway-api/apisx/v1alpha1/backendtrafficpolicy_types.go

Lines 122 to 141 in 2b0b9ce

	// BudgetPercent defines the maximum percentage of active requests that may
	// be made up of retries.
	//
	// Support: Extended
	//
	// +optional
	// +kubebuilder:default=20
	// +kubebuilder:validation:Minimum=0
	// +kubebuilder:validation:Maximum=100
	BudgetPercent *int `json:"budgetPercent,omitempty"`
	// BudgetInterval defines the duration in which requests will be considered
	// for calculating the budget for retries.
	//
	// Support: Extended
	//
	// +optional
	// +kubebuilder:default="10s"
	// +kubebuilder:validation:XValidation:message="budgetInterval can not be greater than one hour or less than one second",rule="!(duration(self) < duration('1s') || duration(self) > duration('1h'))"
	BudgetInterval *Duration `json:"budgetInterval,omitempty"`

/cc @ericdbishop @mikemorris