Support nested arrays of primitive values inside of objects

[czechboy0]

Motivation

It's a useful pattern to define a single JSON schema for all your (e.g. query) parameters, and handle them as a single object in your code.

In OpenAPI, that'd be expressed like this, for example:

# parameter
name: myParams
in: query
explode: true
style: form
schema:
  $ref: '#/components/schemas/QueryObject'

# schema
QueryObject:
  type: object
  properties:
    myString:
      type: string
    myList:
      type: array
      items:
      	type: string

Until now, the myList property would not be allowed, and would fail to serialize and parse, as arrays within objects were not allowed for form style parameters (used by query items, by default).

Modifications

This PR extends the support of the form style to handle single nesting in the top level objects. It does not add support for arbitrarily deep nesting.

As part of this work, we also now allow the deepObject style to do the same - use arrays nested in an object.

Result

The useful pattern of having an array within a "params" object works correctly now.

Test Plan

Added unit tests for all 4 components: encoder, decoder, serializer, and parser.

[simonjbeaumont]

Thanks! One question about whether we need to break out this type.