Filter/Where as a standalone concept

Refactor the current implementation of LoopBack's Filter to allow other packages to leverage the filtering implementation and/or support LoopBack's Filter syntax.

1. The syntax of Filter/Where objects should be well defined and properly documented, to allow 3rd-party implementations to understand all possible values.

2. There should be TypeScript typings describing Filter, Where, etc., these typings must be accessible independently on @loopback/repository (e.g. via a new package).

3. The implementation used by our memory connector should be refactored/extracted into a package that can be used independently from juggler (something like https://www.npmjs.com/package/loopback-filters). It's important to update the connector to use that new package, to ensure there is only single reference implementation.

Nice to have:

• JSON Schema describing Filter syntax
• Some sort of a test suite to allow 3rd-party implementations to verify conformance

Beginne damit, die vorhandene Implementierung von Filter/Where und den Memory-Connector in juggler zu finden, und prüfe anschließend, wie @loopback/repository seine Typen bereitstellt. Als erledigt gilt die Aufgabe, wenn die Syntax dokumentiert ist, unabhängige TypeScript-Typdefinitionen und ein wiederverwendbares Filterpaket vorhanden sind, der Memory-Connector die gemeinsame Implementierung verwendet und die vorgeschlagenen JSON-Schema- oder Konformitätstests berücksichtigt wurden.