Allow passing a schema class instead of schema instances

[sloria]

Passing a schema instance makes it impossible to use schema context safely.

[taion]

Somewhat related to #238.

I guess I was thinking that individual views might want to pass things into schema constructors, but that's never really happened.

Though, in practice, in a Flask context, wouldn't it be nearly equivalent to use flask.g instead of the schema context?

[sloria]

Hm, we could do what DRF does and allow either a schema_class class variable or a get_schema_instance method to be defined.

flask.g could work, though I'd worry if there are cases when schemas need to get instantiated multiple times per-request. Probably not an issue, though, considering we've been using instances this whole time.

[itajaja]

@sloria if multiple schemas need to be instantiated multiple times per-request, then they will have a separate context, so no problem there?

[sloria]

@itajaja If you need a separate context per-instance, then flask.g could be problematic.

That said, I guess you could work around this by setting flask.g in pre_load and then resetting it in post_load.

[taion]

@itajaja is saying that if you have separate instances, then you would in fact be able to set context on them normally.