Add possibility to verify which methods were called

[vbreuss]

Inspired by TestableIO/System.IO.Abstractions#1081:
Add an option to verify which methods were called on the MockFileSystem. This should cover the following use cases:

• Determine if a method was called with specific parameters
• Determine if a method was called before another method
• Determine how often a method was called

Idea:

• Add a Statistics property on the MockFileSystem. All external calls get registered there.
• This property should expose a list of called methods, each with the name of the method and its parameters.
• The calls should contain a global incremented "sort order" to determine the order in which the methods were called.

We could then write extension methods (e.g. in https://github.com/Testably/Testably.Abstractions.FluentAssertions) to verify the above scenarios.

[vbreuss]

It would also be interesting to have statistics for property access (e.g. when a getter or setter was used)

[github-actions]

This is addressed in release v3.1.0.