init ratelimiter backend from config, not code

[ponytailer]

For example:

RateLimiter(
    backend="simple/ratelimiter.backend.simple",
    ...
)

Because, different env config has the same code.

In CI/CD, use the simple, but use the redis in production.

@abersheeran

[abersheeran]

I don't think it's too difficult for users to write their own code to read the configuration.🤔

[ponytailer]

so, one code supports two env and config. How?

[abersheeran]

"less is more"