Item and ManagedEnvelope (contained Context) are massive

[Dav1dde]

Envelope Item and ManagedEvenlope are quite massive nowadays in raw byte sizes.

Generally an Envelope is already boxed inside the ManagedEnvelope but also the context has grown quite large over time.

Item has all of its headers typed out which is quite large, a quick win should be to make this an enum for all headers which can appear together.