Log diagnostic information

[sbomer]

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 xamarin/xamarin-macios#11374 (comment)