Add logging with LibLog

[bgrainger]

LibLog (NuGet) is "a single file for you to either copy/paste or install via nuget, into your library/framework/application to provide a logging abstraction".

A source code package designed primarily for library and framework authors who want dependency free logging support in their component.

[bgrainger]

After getting the LibLog source code, I see that it uses dynamic in a few places, which is potentially bad for performance and brings in unwanted dependencies.

Additionally, it's not clear that the library was developed with performance as a primary concern; ideally, the library should be (close to) zero overhead if logging is disabled. damianh/LibLog#129 discusses this and other issues.

Npgsql had a custom logging implementation, then switched to Microsoft.Extensions.Logging, then reverted that: npgsql/npgsql#1438. They are currently looking at LibLog, and may switch when performance is improved.

For now, I plan to follow their approach and define a custom interface. We could also publish additional NuGet packages for common logging frameworks that automatically hook them up.