Use typing.Annotation for tagging (and remove autofill).

[copybara-service]

Use typing.Annotation for tagging (and remove autofill).

Until this change, there are 2 ways to tag a field in Fiddle: (1) using the
imperative fiddle.set_tags (and
friends)
API, and (2) the data-flow MyTag.new(some_value) approach. Both of these
work well, but it's convenient to have a more declarative API.

This change allows a library to tag fields in a dataclass or parameters to a
function using typing.Annotated. For example:

@dataclasses.dataclass
class MyDataclass:
  x: typing.Annotated[int, MyTag]
  y: typing.Annotated[str, OneTag, TwoTags] = 'why?'


def annotations_on_functions(
  a: typing.Annotated[float, FloatyTag] = 4.2,
):
  ...

Autofill first explored leveraging the typing.Annotation design. The
lessons learned from that are being incorporated into the next-generation
design of tagging.