Provide `SpyLogger` (stub) implementation of `ILogger`

[stijnmoreels]

Is your feature request related to a problem? Please describe.
When testing logging functionality with the ILogger across projects; it's sometimes useful to assert on the written messages 'in-memory' without the whole setup of a fully Serilog infrastructure.

Describe the solution you'd like
The SpyLogger exposes the written messages as a read-only sequence so the tests can assert on the expected written logs.