Enable complex query filters

[pudo]

Riffing off of #366, we've been discussing a more advanced filter mechanism in entity mappings both for queries and for schema assignment. I'm sketching this out here to get some feedback.

The basic idea is to make it possible to compile nested complex queries. This might have to be incompatible to the existing filter, filter_not rules in mappings, so we would be introducing a new query section (which filter and filter_not would be transparently re-written to). The query syntax itself could riff off of the MongoDB syntax, which manages to encapsulate compound queries in JSON.

The second purpose of this enhancement would be to make schema conditional on column contents. For example, we often see tables where companies and people are mixed, and whether a row describes a person or company is determined by a value like individual/entity in one column.

Here's a sketch of how we could address both issues:

query:
  table: zz_donors
  query:
    $and:
      country: xk
      $or:
        political_party:
          $like: "%conservative%"
        district: ["Southern", "Northern"]
  entities:
    donor:
      schema_query:
        Person:
          donor_type: "individual"
        Company:
          donor_type: ["corporate", "business"]

Especially keen for feedback from @uhhhuh @brrttwrks :)

[uhhhuh]

I like this solution to the problem of long unmanageable mappings. Would it be possible and reasonable to also support subqueries for properties? Happened before (customs declarations among others): substitute some conventional value with the one from a dictionary.
Given two sections from a mapping:

tables:
  - table: proceedings_documents
    alias: pr
  - table: constans
    alias: const
joins:
  - left: pr.proceeding_type
    right: const.public_code
filters:
  const.type: "type"
entities:
  case:
    schema: CourtCase
    properties:
      type:
        column: const.description_EN

tables:
  - table: proceedings_documents
    alias: pr
  - table: constans
    alias: const
joins:
  - left: pr.proceeding_category
    right: const.public_code
filters:
  const.type: "category"
entities:
  case:
    schema: CourtCase
    properties:
      category:
        column: const.description_EN

be able to reduce them to something like

tables:
  - table: proceedings_documents
    alias: pr
  - table: constants
    alias: const
entities:
  case:
    schema: CourtCase
    properties:
      type:
        column: const({public_code: pr.proceeding_type, type: "type"}, {description_EN: 1})
      category:
        column: const({public_code: pr.proceeding_form, type: "category"}, {description_EN: 1})