Log diagnostic information

The linker should output some diagnostic information so that consumers of the linker with custom extensions don't need to do it themselves, such as:
- the final configured pipeline, with steps and sub steps
- timing information (total time? time per step?)

If the diagnostics are printed to stdout using `LogMessage`, they will get `Low` importance when the linker runs in MSBuild, and the messages will show up (if using the default logger) when using -verbosity:detailed or higher.

See https://github.com/xamarin/xamarin-macios/pull/11374#discussion_r623445455