Add workflow to automatically open CTS issue on spec changes

[psalz]

As discussed in a recent SYCL WG call, in order to avoid having the CTS and spec go out of sync, I'm proposing to introduce a CI workflow that automatically opens an issue in the SYCL-CTS repository whenever a spec pull request opened, unless it has the editorial label upon creation.

The issue has the title "[Spec change] <title of PR>", and looks like this: KhronosGroup/SYCL-CTS#915

The message can simply be updated in the workflow file in this repository; it's relatively brief right now, let me know if you think it needs more information. I'm also assigning the author of the PR to the issue - not sure if that's something we want to do.

For future reference, here is how the whole thing is set up:

• In order to create a new issue in another repository we need an access token. The best way I've found to generate such a token is through a GitHub app with apropriate permissions.
  • The KhronosGroup organization is the owner of the SYCL Issue Bot application, which @outofcontrol set up for me (thanks!).
  • The app only has access to the SYCL-Docs and SYCL-CTS repositories, and is only allowed to read/write issues.
• The app itself doesn't do anything at all. Instead, it is only used to generate a temporary token through the https://github.com/actions/create-github-app-token action as part of the workflow added in this PR.
  • To do so, the action requires the app's ID as well as a secret private key, both of which are provided as GitHub action variable/secret.
• Once the temporary token is generated, it can then be used by the Create GitHub Issue Action. Note that this is not an official GitHub action, and there are several other options available. I chose this one for its simplicity.

[TApplencourt]

LGTM (and thanks for the explanation! Always nice to understand the magic :) )

[tomdeakin]

Workflow. WG happy to merge.