Add GitHub CodeQL code scanning

[kristapsk]

Add GitHub CodeQL code scanning.

#1648 was already found while testing it.

[github-advanced-security]

This pull request sets up GitHub code scanning for this repository. Once the scans have completed and the checks have passed, the analysis results for this pull request branch will appear on this overview. Once you merge this pull request, the 'Security' tab will show more code scanning analysis results (for example, for the default branch). Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results. For more information about GitHub code scanning, check out the documentation.

[kristapsk]

I would like to merge this sooner than later. We clearly lack manpower, so automating things and improving CI is the way to go, so that machines do more work for JoinMarket.

[MarnixCroes]

cACK
not much downside, if any

[kristapsk]

not much downside, if any

Only downside I can think of is extra CI minute. But that's nothing compared to other tests.