Implement SimpleSchema enum support

[a-hilaly]

Feature Description

Currently the enum marker type is not implemented in SimpleSchema, causing validation errors when users try to use it:

failed to parse field schema for mode: invalid marker key 'enum': unknown marker type: enum

Example usage that fails:

mode: string | enum="debug,info,warn,error" default="info"

The implementation can start here, where the enum marker is defined but not implemented yet:

kro/pkg/simpleschema/markers.go

Lines 40 to 49 in fe88501

	type MarkerType string
	const (
	// MarkerTypeRequired represents the `required` marker.
	MarkerTypeRequired MarkerType = "required"
	// MarkerTypeDefault represents the `default` marker.
	MarkerTypeDefault MarkerType = "default"
	// MarkerTypeDescription represents the `description` marker.
	MarkerTypeDescription MarkerType = "description"
	)

[KekmaTime]

@a-hilaly i would like to work on this issue. can i take it up

[a-hilaly]

@KekmaTime I just assigned it to you, let us know if you run into any issues!