Nested `pw.Schema`

[olruas]

Is your feature request related to a problem? Please describe.
Currently, there is no way to define a nested schema using pw.Schema.

Describe the solution you'd like
Be able to use nested pw.schema:

class NestedColumnSchema(pw.Schema):
    a: int 
    b: float

class InputSchema(pw.Schema):
    key: int
    nested_column: NestedColumnSchema

Describe alternatives you've considered
Currently, a workaround is to use pw.JSON, but the engine cannot verify the schema inside it.

Additional context
Link to issue #117