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>

scripts/release/templates/runtimes.md.tera

@@ -0,0 +1,19 @@
+{# This include shows the list and details of the runtimes #}
+{%- import "runtime.md.tera" as m_r -%}
+
+{# ---  #}
+
+## Runtimes
+
+{% set rtm = srtool[0] -%}
+
+The information about the runtimes included in this release can be found below.
+The runtimes have been built using [{{ rtm.data.gen }}](https://github.com/paritytech/srtool) and `{{ rtm.data.rustc }}`.
+
+{%- for runtime in srtool | sort(attribute="name") %}
+{%- set HIDE_VAR = "HIDE_SRTOOL_" ~ runtime.name | upper %}
+{%- if not env is containing(HIDE_VAR) %}
+
+{{ m_r::runtime(runtime=runtime) }}
+{%- endif %}
+{%- endfor %}