GitHub self-hosted runner setup for macOS

[thomashampson]

Instructions and method for setting up a self-hosted macOS GitHub runner are required.

[RabiyaF]

I can look at this next week but I don't have the permissions to assign myself to the issue.

[RabiyaF]

Sarah has just told me that for security reasons I cannot do the devOps work and only a limited amount of people in the team can do it. So, someone else will have to pick this up.