Add DevSkim scanning

[anjbur]

Adds a new GitHub action to run the Microsoft DevSkim tool. This will integrate with the Security Issues view and scan PRs for new potential issues.

[tcNickolas]

@anjbur Can you clarify what kind of issues are going to be detected and reported, and how is this going to look - is this an extra CI build job that will fail if the PR introduces new issues?

[anjbur]

@tcNickolas It's an additional compliance tool to catch any potential security alerts. It runs a static code analysis and will create a failing PR check if any new issues are introduced by a PR. You can see the added CI checks on this PR: one for the code action to run (DevSkim / DevSkim (pull_request)) and one with the results (Code scanning results / devskim). Any alerts (against main or a PR) are also visible though the Code Scanning Alerts section under the Security tab!

[tcNickolas]

Looks good - I see the two new checks on the pull request passing so we're in a good place to start using them.