Update TokenBucket init to respect user TimeSource

[landonxjames]

Still needs tests, but want to see if it breaks any existing CI first

Motivation and Context

Closes #4459, WASM users were seeing runtime errors because some new functionality added to the TokenBucket was creating a default TimeSource that used SystemTime. WASM is sandboxed and doesn't have access to SystemTime, so this caused a panic.

Description

I updated the TokenBucket creation to initialize the bucket with the user-provided TimeSource from the Config

Added a task to #2499 to update our WASM integration tests to make sure we catch issues like this in the future.

Testing

Added integration test to ensure that the TokenBucket for an operation picks up the TimeSource from the Config if it is set.

Checklist

• 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.
• 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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[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• Client Test (ignoring whitespace)
• No codegen difference in the Server Test
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

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[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• Client Test (ignoring whitespace)
• No codegen difference in the Server Test
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

A new doc preview is ready to view.