Sieve 3 architecture and extensibility points

[Biarity]

Let's try to list some scenarios where Sieve is being used, to try and address any pain points with the current architecture. Breaking changes are OK for the v3.0 release. A part of this is locking down overall scope for Sieve 3.

Example:

• Scenario: using Sieve to filter using a JSON query
• Current solution: developer has to write a custom ISieveModel with a JSON parser
• Better solution: implement a built-in optional JSON-based model