Initialize rate_limit_quota token buckets with a partial fill

[bsurber]

Commit Message: When replacing a token bucket with another in a rate_limit_quota filter's bucket cache, initialize the replacement in a partially filled state, based on the ratio of available and max tokens of the existing token bucket.

Additional Description: This enables more frequent updates in RateLimitQuotaResponses when giving assignments for token buckets. Previously, updates to an assignment had a side effect of resetting the available tokens in the token bucket. This implementation doesn't perfectly account for asynchronous flows but is better than the implementation.

Risk Level: Low - the AtomicTokenBucketImpl class isn't widely used yet & changes are minimal. The rate_limit_quota filter is still in development.
Testing: Integration testing
Docs Changes:
Release Notes:
Platform Specific Features:

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Caused by: #37624 was opened by bsurber.

see: more, trace.