proto: Suppress large AcceptError clippy warning

This commit uses an allow directive to suppress the following error:

```
warning: the `Err`-variant returned from this function is very large
   --> quinn-proto/src/endpoint.rs:525:10
    |
525 |     ) -> Result<(ConnectionHandle, Connection), AcceptError> {
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the `Err`-variant is at least 136 bytes
    |
    = help: try reducing the size of `endpoint::AcceptError`, for example by boxing large elements or replacing it with `Box<endpoint::AcceptError>`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#result_large_err
    = note: `#[warn(clippy::result_large_err)]` on by default
```

Because AcceptError and all of its descendants fields are public,
excepting a descendant that contains a usize, it is impossible to reduce
the byte size of AcceptError without semver breakage.

Author: gretchenfrage

quinn-proto/src/endpoint.rs

@@ -516,6 +516,8 @@ impl Endpoint {
     }
 
     /// Attempt to accept this incoming connection (an error may still occur)
+    // AcceptError cannot be made smaller without semver breakage
+    #[allow(clippy::result_large_err)]
     pub fn accept(
         &mut self,
         mut incoming: Incoming,