Add "Tags" check in POST /signature endpoint

[Marketen]

POST /signatures endpoint doesnt do any check on the "Tag" besides validating that it is not empty. Ideally it should check that is one of the defined ones in tyoes.go:

validator-monitoring/listener/internal/api/types/types.go

Lines 17 to 26 in b87d9b6

	const (
	Obol Tag = "obol"
	Diva Tag = "diva"
	Ssv Tag = "ssv"
	Rocketpool Tag = "rocketpool"
	Stakewise Tag = "stakewise"
	Stakehouse Tag = "stakehouse"
	Solo Tag = "solo"
	Stader Tag = "stader"
	)

Validation could look like:

func isValidTag(tag types.Tag) bool {
	switch tag {
	case types.Obol, types.Diva, types.Ssv, types.Rocketpool, types.Stakewise, types.Stakehouse, types.Solo, types.Stader:
		return true
	default:
		return false
	}
}