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/changes.md.tera

@@ -0,0 +1,4 @@
+{# This include generates the section showing the changes #}
+## Changelog
+
+{% include "changelog.md" -%}

scripts/release/templates/compiler.md.tera

@@ -0,0 +1,6 @@
+## Rust compiler versions
+
+This release was built and tested against the following versions of `rustc`.
+Other versions may work.
+
+- Rust Stable: `{{ env.RUSTC_STABLE }}`

scripts/release/templates/runtime.md.tera

@@ -0,0 +1,26 @@
+{# This macro shows one runtime #}
+{%- macro runtime(runtime) -%}
+
+### {{ runtime.name | title }}
+
+{%- if runtime.data.runtimes.compressed.subwasm.compression.compressed %}
+{%- set compressed = "Yes" %}
+{%- else %}
+{%- set compressed = "No" %}
+{%- endif %}
+
+{%- set comp_ratio = 100 - (runtime.data.runtimes.compressed.subwasm.compression.size_compressed /
+runtime.data.runtimes.compressed.subwasm.compression.size_decompressed *100) %}
+
+```
+🏋️ Runtime Size:          {{ runtime.data.runtimes.compressed.subwasm.size | filesizeformat }} ({{
+runtime.data.runtimes.compressed.subwasm.size }} bytes)
+🔥 Core Version:          {{ runtime.data.runtimes.compressed.subwasm.core_version.specName }}-{{runtime.data.runtimes.compressed.subwasm.core_version.specVersion }} ({{runtime.data.runtimes.compressed.subwasm.core_version.implName }}-{{runtime.data.runtimes.compressed.subwasm.core_version.implVersion }}.tx{{runtime.data.runtimes.compressed.subwasm.core_version.transactionVersion }}.au{{runtime.data.runtimes.compressed.subwasm.core_version.authoringVersion }})
+🗜 Compressed:            {{ compressed }}: {{ comp_ratio | round(method="ceil", precision=2) }}%
+🎁 Metadata version:      V{{ runtime.data.runtimes.compressed.subwasm.metadata_version }}
+🗳️ system.setCode hash:   {{ runtime.data.runtimes.compressed.subwasm.proposal_hash }}
+🗳️ authorizeUpgrade hash: {{ runtime.data.runtimes.compressed.subwasm.parachain_authorize_upgrade_hash }}
+🗳️ Blake2-256 hash:       {{ runtime.data.runtimes.compressed.subwasm.blake2_256 }}
+📦 IPFS:                  {{ runtime.data.runtimes.compressed.subwasm.ipfs_hash }}
+```
+{%- endmacro runtime %}

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 %}

scripts/release/templates/template.md.tera

@@ -0,0 +1,9 @@
+{# This is the entry point of the template -#}
+
+This release contains the changes from `{{ env.REF1 | replace(from="refs/tags/", to="") }}` to `{{ env.REF2 | replace(from="refs/tags/", to="") }}`.
+
+{% include "changes.md.tera" -%}
+
+{% include "compiler.md.tera" -%}
+
+{% include "runtimes.md.tera" -%}