Variant: create variant that holds a variant

[tmds]

The D-Bus spec allows a variant to contain another variant. This is not a common use-case.

This issue is for adding support to represent these wrapped variants using the Tmds.DBus.Protocol.Variant type.

Adding a public constructor may be confusing because the user may expect copy value semantics.

Proposed API:

struct Variant
{
  public static Variant WrapVariant(Variant variant);