feat(server): backport max_pending_accept_reset_streams() to builder (#3796)

the `server::conn::http2` submodule provides types that are
conditionally compiled when the `backports` feature is active, to
facilitate upgrading to the `1.x` release.

i've been working on upgrading the
[`linkerd2-proxy`](https://github.com/linkerd/linkerd2-proxy/) project
to use hyper 1.0, but encountered some gaps in `Builder<E>`'s interface
when setting the `backports` and `deprecated` features.

this adds a `max_pending_accept_reset_streams(..)` method
(_added in #3201_) to this builder, so that 0.14.x users relying on
this functionality can prepare to upgrade to hyper 1.x safely.

this is effectively a backport of #3507, which restored this interface
to the 1.0 release.

for more information, see:

* #3201
* #3507
* #3461
* https://hyper.rs/guides/1/upgrading/

Signed-off-by: katelyn martin <me+cratelyn@katelyn.world>

Author: cratelyn

src/server/conn/http2.rs

@@ -175,6 +175,17 @@ impl<E> Builder<E> {
         self
     }
 
+    /// 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).
+    /// As of v0.3.17, it is 20.
+    ///
+    /// See <https://github.com/hyperium/hyper/issues/2877> for more information.
+    pub fn max_pending_accept_reset_streams(&mut self, max: impl Into<Option<usize>>) -> &mut Self {
+        self.h2_builder.max_pending_accept_reset_streams = max.into();
+        self
+    }
+
     /// Sets an interval for HTTP2 Ping frames should be sent to keep a
     /// connection alive.
     ///