Add `check runtimes` GH Workflow (#2252)

This PR introduces:
- a new script 
- a new GH Workflow 
- runtime reference spec files for `rococo` and `westend`

It brings a mechanism to check that part(s) of the runtimes' metadata
that should not change over time, actually did not change over time.

Ideally, the GHW should trigger when a release is edited but GH seem to
[have an issue](https://github.com/orgs/community/discussions/47794)
that prevents the trigger from working. This is why the check has been
implemented as `workflow_dispatch` for a start.

The `workflow_dispatch` requires a `release_id` that can be found using:
```
curl -s -H "Authorization: Bearer ${GITHUB_TOKEN}" \
   https://api.github.com/repos/paritytech/polkadot-sdk/releases | \
   jq '.[] | { name: .name, id: .id }'
```
as documented in the workflow.

A sample run can be seen
[here](https://github.com/chevdor/polkadot-sdk/actions/runs/6811176342).