[OpenAPI] Extract the JSON Schema builder

[dunglas]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	#2715, #2415, #2502, api-platform/api-platform#1222
License	MIT
Doc PR	todo

Introduce a new service allowing to generate a JSON Schema from any class, including those marked with @ApiResource.

It's an alternative approach to #2715 to support specific JSON Schemas per MIME type in OpenAPI (this part is not ready yet).
It will also allow to generate JSON Schema directly, without having to rely on OpenAPI. It can be practical for validation, or to use libraries such as https://github.com/mozilla-services/react-jsonschema-form

This new capability will also replace https://github.com/dunglas/php-to-json-schema

TODO:

• Add support for formats

[dunglas]

I added support for the writeOnly and deprecated fields defined by the JSON Schema spec. I also refactored heavily the code.

[dunglas]

AppVeyor failure related to symfony/symfony#32879 (fixed in 4.3-dev). Ignoring for now.

[dunglas]

DocumentationNormalizer can be simplified a bit more, and unit tests must be added for the new classes, but it works!