Sourcing service config from the environment. (#3493)

## 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 -->
https://github.com/smithy-lang/smithy-rs/issues/2863
https://github.com/awslabs/aws-sdk-rust/issues/1060

## Description
<!--- Describe your changes in detail -->
This PR adds a new feature: the ability to source service-specific
config from the environment.
This is **only** supported when creating a service config from an
`SdkConfig`. I've posted [a
guide](https://github.com/smithy-lang/smithy-rs/discussions/3537) to our
discussions board.

[This also adds support for setting an endpoint URL in environment
config.](https://github.com/smithy-lang/smithy-rs/issues/2863

)

## 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. -->
I have written several tests ensuring config is extracted with the
correct precedence.

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

----

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choice._

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Co-authored-by: John DiSanti <jdisanti@amazon.com>
Co-authored-by: ysaito1001 <awsaito@amazon.com>