tuf-0.3.0-beta11 fails to build against ToT Rust toolchain

[amanda-tait]

Beginning with rust-lang/rust 42752cbe, tuf fails to build with the following error message:

error[E0733]: recursion in an async fn requires boxing
   --> ../../third_party/rust_crates/vendor/tuf-0.3.0-beta11/src/client.rs:955:5
    |
955 | /     async fn lookup_target_description(
956 | |         &mut self,
957 | |         start_time: &DateTime<Utc>,
958 | |         default_terminate: bool,
...   |
962 | |         targets: Option<(&Verified<TargetsMetadata>, MetadataPath)>,
963 | |     ) -> (bool, Result<TargetDescription>) {
    | |__________________________________________^
    |
    = note: a recursive `async fn` call must introduce indirection such as `Box::pin` to avoid an infinitely sized future