fix token bucket not being set for both standard and adaptive retry modes (#3964)

## Motivation and Context
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<!--- If it fixes an open issue, please link to the issue here -->
https://github.com/awslabs/aws-sdk-rust/issues/1234

## Description
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PR adds a new interceptor registered as part of the default retry plugin
components that ensures a token bucket is _always_ present and available
to the retry strategy. The buckets are partitioned off the retry
partition (which defaults to the service name and is already set by the
default plugin). We use a `static` variable in the runtime for this
which means that token buckets can and will apply to every single client
that uses the same retry partition. The implementation tries to avoid
contention on this new global lock by only consulting it if the retry
partition is overridden after client creation.

For AWS SDK clients I've updated the default retry partition clients are
created with to include the region when set. Now the default partition
for a client will be `{service}-{region}` (e.g. `sts-us-west-2`) rather
than just the service name (e.g. `sts`). This partitioning is a little
more granular and closer to what we want/expect as failures in one
region should not cause throttling to another (and vice versa for
success in one should not increase available quota in another).

I also updated the implementation to follow the SEP a little more
literally/closely as far as structure which fixes some subtle bugs.
State is updated in one place and we ensure that the token bucket is
always consulted (before the token bucket could be skipped in the case
of adaptive retries returning a delay and the adaptive rate limit was
updated in multiple branches).


## Testing
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to -->
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## Checklist
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rather than leaving it unchecked -->
- [x ] For changes to the smithy-rs codegen or runtime crates, I have
created a changelog entry Markdown file in the `.changelog` directory,
specifying "client," "server," or both in the `applies_to` key.
- [ x] For changes to the AWS SDK, generated SDK code, or SDK runtime
crates, I have created a changelog entry Markdown file in the
`.changelog` directory, specifying "aws-sdk-rust" in the `applies_to`
key.

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