Restore `SmokeTestsDecoratorTest` (#3811)

## Motivation and Context
Restores `SmokeTestsDecoratorTest` that was temporarily removed in
https://github.com/smithy-lang/smithy-rs/pull/3808.

## Description
The said test was temporarily removed because `SmokeTestsDecorator` is
included in the [predefined
decorators](https://github.com/smithy-lang/smithy-rs/blob/b38ccb969e09ae0856c235fcb496b3f3faf41c87/aws/sdk-codegen/src/main/kotlin/software/amazon/smithy/rustsdk/AwsCodegenDecorator.kt#L35)
and pulls-in the `aws-config` crate for code generation. The issue was
conflicts between the runtime crates used by `aws-config` (located in
the `aws/sdk/build` directory) and those brought-in by that predefined
decorators used by `awsSdkIntegrationTest` (located in the
`rust-runtime` and `aws/rust-runtime` directories). This PR addresses
these conflicts and restores the test functionality.

Given the challenges, the code changes are centered around the following
ideas:
- Focus solely on testing the core class, `SmokeTestsInstantiator`. By
doing this, we avoid running `SmokeTestsDecorator`, which would
otherwise pull in the `aws-config` crate.
- Initializing a config builder in smoke tests needs to be parameterized
depending on the environment; in production we use
`aws_config::load_defaults` and in testing we avoid including
`aws-config` by using a default-constructed config builder, which is
sufficient for compile-only tests.
- The generated smoke tests in `SmokeTestsDecoratorTest` require minimal
runtime crates for compilation. We need the `CodegenVisitor` but with a
different set of codegen decorators. Prior to this PR,
`awsSdkIntegrationTest` used
[RustClientCodegenPlugin](https://github.com/smithy-lang/smithy-rs/blob/b38ccb969e09ae0856c235fcb496b3f3faf41c87/codegen-client/src/main/kotlin/software/amazon/smithy/rust/codegen/client/smithy/RustClientCodegenPlugin.kt#L46)
that in turn loaded [predefined
decorators](https://github.com/smithy-lang/smithy-rs/blob/b38ccb969e09ae0856c235fcb496b3f3faf41c87/aws/sdk-codegen/src/main/kotlin/software/amazon/smithy/rustsdk/AwsCodegenDecorator.kt#L35)
on the classpath, which included `SmokeTestsDecorator`. In this PR, we
work around this by defining a minimal set of codegen decorators and
making them pluggable through `awsSdkIntegrationTest`.

## Testing
Tests in CI

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