[Feature Request]: User-facing API for defining schema/field options on user types

### What would you like to happen?

We should have a user-facing API for defining beam schema options on user types. One concept might be a `@beam_option` decorator that can be applied to a class (schema-level metadata) or an attribute (field-level metadata).

```py
@beam_option("my_flag")
@beam_option("my_metadata", 123)
class MyRowType(NamedTuple):
@beam_option("my_field_flag")
foo: int
@beam_option("my_metadata", 456)
bar: str
```

### Issue Priority

Priority: 2

### Issue Component

Component: sdk-py-core