api: add a standard -dump-requests flag

Created by: LawnGnome

This flag dumps the request and response out to stdout for any request issued via the internal/api package. This is probably only useful to people developing src-cli, but it's also pretty lightweight.

The output is very basic right now, but that's mostly because we have lots of implementation details stuck unexported in cmd/src that would be useful for colouring. If we land this and #268, then I'll likely rework this to use the new Output type.