feat(http2): add Builder::executor

`Builder::new(exec)` is the only constructor on either HTTP/2 builder and the
executor cannot be read back, so a caller that needs it for work alongside a
connection keeps a second copy with nothing relating the two.

`impl<E> Builder<E>` is unconstrained, so the accessor needs no bound, and
returning `&E` leaves cloning to the caller.

Author: robertream

src/client/conn/http2.rs

@@ -314,6 +314,11 @@ where
         }
     }
 
+    /// Returns the executor this builder was created with.
+    pub fn executor(&self) -> &Ex {
+        &self.exec
+    }
+
     /// Provide a timer to execute background HTTP2 tasks.
     pub fn timer<M>(&mut self, timer: M) -> &mut Builder<Ex>
     where

src/server/conn/http2.rs

@@ -120,6 +120,11 @@ impl<E> Builder<E> {
         }
     }
 
+    /// Returns the executor this builder was created with.
+    pub fn executor(&self) -> &E {
+        &self.exec
+    }
+
     /// Configures the maximum number of pending reset streams allowed before a GOAWAY will be sent.
     ///
     /// This will default to the default value set by the [`h2` crate](https://crates.io/crates/h2).