❇️ Re-implementation of plugins API

[pleonex]

This PR introduces a new design of the plugins API. The classes are now in the new library Yarhl.Plugins. This implementation does not use any underlying technology like MEF in the past.

There are two main classes:

• TypeLocator: it scans the current loaded assemblies to find types that implement the given interface.
• ConverterLocator using TypeLocator keeps a cache of types and converters found in assemblies.

Additionally, new extension methods for AssemblyLoadContext help to load assemblies from the current executing directory or a given path. By default it does not load any additional assembly from any folder anymore. This was a potential security risk. Developers can choose to do it via these extension methods, alerting the user first if wanted.

Quality check list

• Related code has been tested automatically or manually
• Related documentation is updated
• I acknowledge I have read and filled this checklist and accept the
  developer certificate of origin

Acceptance criteria

• The library does not load additional assemblies by default
• The library can find formats in the loaded assemblies
• The library can find converters in the loaded assemblies
• Developers can load assemblies from additional paths with APIs

Follow-up work

None

Example

• Find converters

var converters = ConverterLocator.Instance.Converters;

• Find formats

var formats = ConverterLocator.Instance.Format;

• Find a type that implements the given interface

var types1 = TypeLocator.Instance.FindImplementationOf(typeof(IFormat));

// For generic types use this overload
var types2 = TypeLocator.Instance.FindImplementationOfGeneric(typeof(IConverter<,>));