scxtop: Add trace command

[hodgesds]

Add a trace subcommand for scxtop to generate perfetto traces from the CLI without using the TUI mode. Tracing and TUI interactions are separate enough in their use cases that there should be a dedicated way of generating traces. The existing TUI integration should be kept for generating traces, however the trace subcommand should follow a separate code path. This may require a bit of refactoring to support subcommands with args for enabling/disabling various events.