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>

.github/scripts/common/lib.sh

@@ -369,7 +369,7 @@ function relative_parent() {
 # used as Github Workflow Matrix. This call is exposed by the `scan` command and can be used as:
 # podman run --rm -it -v /.../fellowship-runtimes:/build docker.io/chevdor/srtool:1.70.0-0.11.1 scan
 function find_runtimes() {
-    libs=($(git grep -I -r --cached --max-depth 20 --files-with-matches 'construct_runtime!' -- '*lib.rs'))
+    libs=($(git grep -I -r --cached --max-depth 20 --files-with-matches '[frame_support::runtime]!' -- '*lib.rs'))
     re=".*-runtime$"
     JSON=$(jq --null-input '{ "include": [] }')
 
@@ -434,3 +434,13 @@ check_release_id() {
   fi
 
 }
+
+# Get latest release tag
+#
+# input: none
+# output: latest_release_tag
+get_latest_release_tag() {
+    TOKEN="Authorization: Bearer $GITHUB_TOKEN"
+    latest_release_tag=$(curl -s -H "$TOKEN" $api_base/paritytech/polkadot-sdk/releases/latest | jq -r '.tag_name')
+    printf $latest_release_tag
+}