Add serde option to @rust.Adapter annotation

Summary:
The Rust generator has validations to disallow the combination of adapter + serde on the same type. However, those validations are not currently enforced correctly - the Rust generator initializes its options in `generate_program`. This means the options are *not* initialized when `validate_program` runs.

https://www.internalfb.com/code/fbsource/[017165f964e6]/xplat/thrift/compiler/compiler.cc?lines=526-536

This means the Rust validators don't get the true view of options, which currently allows invalid programs to pass validation.

E.g. `validate_struct_annotations` disallows the `rust.Adapter` annotation with the `serde` compiler/generator option, but because `rust_codegen_options::serde` is not initialized until `generate_program` is called, `serde` will always be `false` when the validator runs.

This adds a new `serde` option on the `rust.Adapter` annotation, which can be set to inform the compiler/generator "this adapter will implement Serialize/Deserialize traits". A follow-up change will update the validation to allow `rust.Adapter` in combination with serde if this option is set to true.

Reviewed By: dtolnay

Differential Revision: D98007932

fbshipit-source-id: 6ce3eb479fbc26d749ff817cc76d81587c47e419

Author: hchokshi

thrift/annotation/rust.thrift

@@ -344,6 +344,17 @@ struct Mod {
 @scope.Struct
 struct Adapter {
   1: string name;
+  /**
+   * If true, the adapter's `AdaptedType` is expected to implement
+   * `serde::Serialize` and `serde::Deserialize`. This allows the adapted field
+   * to be used in types that derive serde traits (via the `serde` codegen
+   * option or `@rust.Serde`).
+   *
+   * If false (default), using this adapter on a field with serde enabled will
+   * produce a validation error, since the compiler cannot verify that the
+   * `AdaptedType` implements the required serde traits.
+   */
+  2: bool serde = false;
 }
 
 /**