Add a convenience script for consistent astyle formatting

[SWilson4]

The behaviour of astyle varies across versions and platforms, which means it can sometimes be a pain to get PRs to pass CI formatting checks. This PR adds a convenience script which runs astyle over local code in the CI Docker image, ensuring that local formatting matches what's expected by CI.

My hope is that this script will reduce friction for new contributors and busywork for regular ones. I've been using this approach for a while myself.

• Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in fully supported downstream projects dependent on these, i.e., oqs-provider will also need to be ready for review and merge by the time this is merged.)

[baentsch]

A very sensible improvement for any current and new contributor, Thanks @SWilson4 !

Re-reading the CONTRIBUTING.md file also gave me thought for some more improvements...