Github workflow to automate release draft creation (#3978)

This PR introduces the github flow which will create a release draft
automatically when the rc tag is pushed. The flow contains the following
steps:

- Gets the info about rust version used to build the node
- Builds the runtimes using `srtool`
- Extracts the info about each runtime 
- Aggregates the changelog from the prdocs
- Creates the release draft containing all the info related to the
release (changelog, runtimes, rust versions)
- Attaches the runtimes to the draft
- Posts the message to the RelEng internal channel to inform that the
build is done.

Related to the #3295

---------

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

prdoc/1.9.0/pr_3513.prdoc

@@ -5,7 +5,13 @@ title: Fix call enum's metadata regression
 
 doc:
   - audience: Runtime Dev
-  - audience: Runtime User
+    description: |
+      This PR fixes an issue where in the metadata of all FRAME-based chains, the documentation for
+      all extrinsic/call/transactions has been replaced by a single line saying "see Pallet::name".
+
+      Many wallets display the metadata of transactions to the user before signing, and this bug
+      might have affected this process.
+  - audience: [Runtime User, Runtime Dev]
     description: |
       This PR fixes an issue where in the metadata of all FRAME-based chains, the documentation for
       all extrinsic/call/transactions has been replaced by a single line saying "see Pallet::name".