Add workflow for auto updating submodules and submitting a PR

[agorararmard]

This simple workflow will run whenever there is a push to master and with a corn job. It will update all submodules to the latest commit in their remote and submit a PR to master from the auto-created/auto-deleted branch.

• [ ] Tests pass
• [ ] Appropriate changes to README are included in PR

[umarcor]

@agorararmard, if I don't get it wrong, this is already supported by https://dependabot.com/. It used to be a (free) third-party service, but it was then bought by GitHub. It's now a built-in service and it's the tool used internally for tracking security/critical update warnings.

Therefore, I don't think a custom workflow is necessary. Maintainers just need to enable it in the settings.

[mithro]

@umarcor For various complicated reasons, we need to use a custom action here.

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