[dotnet] Possibility to output internal log messages to file

[nvborisenko]

Continue improvement of internal logging.

Description

• Added FileLogHandler class which listens to internal log events
• Fixed issue when copying of log handlers across contexts

Motivation and Context

To be aligned with others bindings, and providing file log handler out of the box.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[nvborisenko]

This change introduces a breaking change to ILogHandler. I even don't mind to land this feature as v4.16.2. Then docs will be aligned across bindings.

Why now rather than later?
Stabilize public API and avoid breaking changes in future.

[nvborisenko]

Thanks @nvborisenko for your contribution!