Implement Protobuf handling of Unknown

[andrewmwells-amazon]

Describe the improvement you'd like to request

In impl From<&Expr> for proto::Expr, the case for ExprKind::Unknown is unimplemented. This should be fixed before the protobufs feature is stabilized.

Describe alternatives you've considered

NA

Additional context

No response

Is this something that you'd be interested in working on?

• [ ] 👋 I may be able to implement this internal improvement
• [ ] ⚠️ This feature might incur a breaking change