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/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 }}`