Change @rust.Serde default to true

Summary:
For most Rust annotations (`rust.Ord`, `rust.Copy`, `rust.Exhaustive`) the annotation name conveys an opt-in. Opt in to `Ord` impls, `Copy` impls, exhaustive matching, etc.

The `rust.Serde` annotation confusingly did not work like this. `rust.Serde` was treated equivalently to `rust.Serde{enabled = false}`, which is an opt-out for use inside of Thrift targets configured with `thrift_rust_options = "serde"` in TARGETS.

This diff changes `rust.Serde` to mean `rust.Serde{enabled = true}` and leaves `rust.Serde{enabled = false}` for the rare opt-out.

Reviewed By: zertosh

Differential Revision: D69065617

fbshipit-source-id: 439589297f6a97b9d3efd2d59a38f6e45d726d53

thrift/annotation/rust.thrift

@@ -275,7 +275,7 @@ struct Type {
 @scope.Enum
 @scope.Union
 struct Serde {
-  1: bool enabled;
+  1: bool enabled = true;
 }
 
 // # `rust.Mod`