[9.x] Make throttle lock acquisition retry time configurable

[timmartin19]

Depending on context, 750ms can be a long time to wait between retries
for a throttle. We are making the retry time configurable so that users
can tune the throttling further.

All changes are backwards compatible.

I set the unit as milliseconds because this seemed like the natural time
unit to use. Seconds would require floats which felt dirty. That being
said, it's not consistent with the unit used for ->every and ->block
on the builder. So I'm open to changing it for consistency.

I was additionally unsure what useful tests could be written for this
configuration. Mocking the usleep function felt silly. Any suggestions
are welcome.

Redis::throttle('my-key')
    ->allow(5)
    ->every(10)
    ->retryWait(100)
    ->then(static fn () => echo "hello!");

[taylorotwell]

Renamed method to sleep.