Implement ExternalValue

[Caellian]

This is initial implementation that adds support for abstract-types.

Not well tested (or at all).

I'm mostly creating this to provide an exhaustive example that compiles.

Could close #93 with some additional work, or be used for top level Value trait mentioned in #76.

[Caellian]

After reading through spec a little. Abstract values are opaque and their members can't be accessed. The added ExternalValue struct allows working in both opaque and non-opaque modes, so I just documented this distinction.

It will default to non-opaque mode if type implements AsValue. Otherwise, an opaque value can always be constructed.

Also, the drop logic I used is something I already tested in another crate with Miri, so it should work.

Oh, and CEL is designed for use with protobuf, so what #81 is doing might be a better fit. I'll probably fork off with these changes into a different crate if I get really invested in this because my use case calls for a lot of things that go against or aren't aligned with the spec.