add alert details using slack block kit

[russellbrooks]

Adds more details to the slack webhook alerts sent when using --notify-on-error or --notify-on-success using Slack's block kit as part of the argo workflow template creation

Before the messages would simply be:

🚨 TestFlow/argo-testflow-z7mtw failed!

Now they include a link to the UI and conditionally add @project metadata when available, e.g. triggering an argo workflow created from the following:

from metaflow import FlowSpec, project, step


@project(name="test")
class TestFlow(FlowSpec):
    @step
    def start(self):
        self.next(self.end)

    @step
    def end(self):
        pass


if __name__ == "__main__":
    TestFlow()