Add HRMP notification handlers to the xcm-executor (#3696)

Currently the xcm-executor returns an `Unimplemented` error if it
receives any HRMP-related instruction.
What I propose here, which is what we are currently doing in our forked
executor at polimec, is to introduce a trait implemented by the executor
which will handle those instructions.

This way, if parachains want to keep the default behavior, they just use
`()` and it will return unimplemented, but they can also implement their
own logic to establish HRMP channels with other chains in an automated
fashion, without requiring to go through governance.

Our implementation is mentioned in the [polkadot HRMP
docs](https://arc.net/l/quote/hduiivbu), and it was suggested to us to
submit a PR to add these changes to polkadot-sdk.

---------

Co-authored-by: Branislav Kontur <bkontur@gmail.com>
Co-authored-by: command-bot <>

prdoc/pr_3696.prdoc

@@ -0,0 +1,17 @@
+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: Add HRMP notification handlers to the xcm-executor
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      Adds optional HRMP notification handlers to the xcm-executor. These handlers are 3 new config types on the xcm-executor `Config` trait:
+      - `HrmpNewChannelOpenRequestHandler`
+      - `HrmpChannelAcceptedHandler`
+      - `HrmpChannelClosingHandler`
+
+      The traits of these config types are implemented on tuples, and on `()` for the default case.
+
+crates:
+    - name: staging-xcm-executor