Update deployment SDK to use slugs

[desertaxle]

Allows the setting of schedule slugs via a prefect.yaml file:

schedules:
    - cron: "* * * * *"
      slug: "my-important-schedule"

or via the Python SDK:

from prefect import flow
from prefect.schedules import Cron

@flow
def say_hello(name: str = "Marvin"):
    print(f"Hello, {name}")
    
if __name__ == "__main__":
    say_hello.serve("frequently", schedule=Cron("* * * * *", slug="my-important-schedule")

[codspeed-hq]

CodSpeed Performance Report

Merging #17043 will not alter performance

_{Comparing deployment-schedule-sdk-updates (ea20178) with main (b1284f2)}

Summary

✅ 2 untouched benchmarks

[cicdw]

this will prob fail 3.9 checks

[cicdw]

errr or maybe not?

[desertaxle]

The from __future__ import annotations at the top does some black magic that makes this work in 3.9.

[cicdw]

straightforward, LGTM