Improve `QueryFilter`/`Where`/`Filter` Implementation and Documentation

[acupofjose]

Several issues have been raised surrounding the use of QueryFilters with Postgrest.

Notably, confusion around the CONTAINS operator vs. the IN operator and issues with QueryFilters not casting criterion in an expected way.

Suggestions:

• Provide different implementations for IPostgrestQueryFilter for basic queries vs. advanced queries. For instance, the groups of switch operators here should likely be extracted into different classes.
• Improve documentation for the CONTAINS operator (as most cases seem to be expecting the IN or LIKE operator)