Add GitHub Advanced Security requirements

[bensinclair]

Change Description

This PR adds the requirements to integrate GitHub Advanced Security. It includes:

• Actions workflow file that performs the tests when PR's are merging into master branch
• Pslam added to Composer
• Pslam configuration file

We have documentation on what to expect once merged.

Testing

To ensure this works as expected, we should see a successful Analyse PHP test pass below.

Merge

• PR is against the correct branch
• All tests/checks pass

[bensinclair]

Does not support PHP 7.4