Add support for pulling docker images in crank jobs

[davidfowl]

Crank supports launching executables, building docker images etc we should support pulling docker images directly on the agent.

[sebastienros]

We have docker build support with the dockerFile crank argument, docker load support with the dockerLoad crank argument, so the action should be to have a docker pull support with a dockerPull crank argument.

The code would go there:
https://github.com/dotnet/crank/blob/main/src/Microsoft.Crank.Agent/Startup.cs#L2082