Flask-rest-jsonapi-next

- feat: allow injecting page parameters into `Querystring` instances
This allows client app to override pagination properties in special cases, while still
respecting app config everywhere else.
Example for this: app wants to generate Excel file as response and ignore pagination
in that request, but still use JSON:API request parsing. In this case, app would
implement it's own `Resource.get()` method which would mostly the same thing as base
class one, except it would temporarily disable pagination for given request.

- feat: SQLAlchemy data layer returns `Query` objects instead of lists
Speeds up large data requests (ie. when application requests JSON:API data but
disables pagination)

- full indepenedance on which JSON library is used
- use of custom JSON implementation (ie. `rapidjson`, `simplejson`) is now fully
controlled by `flask`:
- we use `flask.json.dumps` everywhere and then the app itself configures what this
`dumps` call does - uses standard JSON library or custom one

- swithced CI provider, Travis --> GitHub
- supporting Flask only >= 2.2.0
- Flask dropped metaclasses in favor of `__init_subclass__` (which is great) but
our `Resource` class relied on metaclasses too
- supporting both mechanisms, metaclasses and `__init_subclass__`, is not feasible,
thus dropping support for older flask versions

- fix: PATCH was not rising ValidationError for fields with `required=True`, when those
fields were missing from `request.body`

- fix: sparse fields were not applied correctly

- feat: added `register_at` parameter to `Api.__init__()`
- fix: `Api` shouldn't use `Flask.config.APLICATION_ROOT` for registering blueprint;
this variable was not meant for that purpose