Allow tagging/labeling client requests

[battis]

This would allow apps that show activity indicators (e.g. https://github.com/groton-school/planner-lti/tree/0462a1102de0beb25098ad836f03b19f703897a0/src/UI/Services/Activity) to send requests to clients and differentiate their activity indicator between, say, a foreground (blocking) process and a background (preloading) process.