Add Schema.from_dict for generating schemas from dicts

[sloria]

close #1312

[lafrech]

LGTM.

Maybe another argument could be added to pass meta options in the call to from_dict but it can be added later on if someone needs it.

[sloria]

I wanted to keep the API surface to a minimum here. If a user needs to customize Meta, they can declare a class.

[lafrech]

Note: from_dict only generates fields for top-level fields in the dict. It does not create Nested fields automatically: https://stackoverflow.com/questions/58190523/how-to-use-schema-from-dict-for-nested-dictionaries.

[sloria]

It was not the intention to automagically create Nested and List fields for dicts and lists. I don't think it's necessary nor intuitive, but I'm not closed off to the idea if others think it's the way to go.

[hardikkat24]

@sloria Wouldn't it be great for fields.Nested() to accept both dictionary and Schema? Thanks!

[sloria]

@hardikkat24 That can be achieved quite easily with a subclass of Nested. See https://github.com/marshmallow-code/webargs/blob/0357a525118acbc19bc5a1981f6161ee79110579/src/webargs/fields.py#L26-L40