Add filter to SortQuery search model

Description:

• Introducing filter field to SortQuery to be able to implement changes for osdu/platform/system/search-service#106 (closed) Changes are backward-compatible, queries without filter are still valid.

How to test:

via Unit test

Changes include:

• Breaking change (a change that is not backward-compatible and/or changes current functionality).

Changes in:

• Common code

Dev Checklist:

• Added Unit Tests, wherever applicable.
• Updated the Readme, if applicable.
• Existing Tests pass
• Verified functionality locally
• Self Reviewed my code for formatting and complex business logic.