Adding Custom Rates

[CahidArda]

With this change, it is now possible to rate limit with a custom rate:

from upstash_ratelimit import Ratelimit, FixedWindow
from upstash_redis import Redis

ratelimit = Ratelimit(
    redis=Redis.from_env(),
    limiter=FixedWindow(max_requests=10, window=10),
)

# pass rate as 5 to subtract 5 from the number of
# allowed requests in the window:
identifier = "api"
response = ratelimit.limit(identifier, rate=5)

Copied the lua scripts from the Typescript ratelimit repository. We had fixed some bugs there so I copied from there to have parity.

[linear]

DX-899 Ratelimit Python SDK: Custom Rates and Get Remainng