Set `pool_idle_timeout` at 90 seconds for default hyper client (#4284)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->
Fixes: https://github.com/smithy-lang/smithy-rs/issues/4282

## Description
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Updating our default Hyper client to be configured with a
[pool_timer](https://docs.rs/hyper-util/latest/hyper_util/client/legacy/struct.Builder.html#method.pool_timer)
and a
[pool_idle_timeout](https://docs.rs/hyper-util/latest/hyper_util/client/legacy/struct.Builder.html#method.pool_idle_timeout)
value of 90 seconds.

Prior to Hyper going 1.0 and the legacy client moving into its own crate
a `pool_timer` was not required. Now the pool won't timeout idle
connections without one. The default value for the timeout in Hyper 0.14
was [90
seconds](https://docs.rs/hyper/0.14.32/hyper/client/struct.Builder.html#method.pool_idle_timeout),
so we are copying that in our current settings.

## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->
This is difficult to test at the moment because Hyper does not support
plugging in your own pool implementation, so we cannot instrument the
pool itself. There has been some movement towards allowing that, so when
that happens we could add tests for this behavior. For the time being we
will have to be satisfied that the Hyper client tests this itself.

## 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.


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