Go 1.18 Generics

[pkasila]

Is your feature request related to a problem? Please describe.
Go 1.18 brings generics, so it would be nice to use them for EncryptedValue and SignedValue.

Describe the solution you'd like
Update EncryptedValue and SignedValue to:

type EncryptedValue[T any] struct {
	Raw T `json:"Raw"`
}
type SignedValue[T any] struct {
	Raw   T           `json:"Raw"`
	Valid bool        `json:"Valid"`
}