re-add support for sourcing endpoint URLs from service-specific env config

[Velfi]

The service env config work I did was flawed in that it didn't respect the precedence of programmatic config. This PR
adds a way of tracking the precedence up to the point of converting the SdkConfig into a service config. Now, env config will only be resolved when config was not set programmatically. I added tests to aws-config for the origin tracking to ensure it works.

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• AWS SDK (ignoring whitespace)
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• AWS SDK (ignoring whitespace)
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• No codegen difference in the Server Test
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[aajtodd]

correctness: Do we want this to be mutable after it is built into SdkConfig?

[Velfi]

Good question. I think so, but I'm not sure. For now, I'll make it so that:

• SdkConfig's Builder exposes an insert_origin method.
• SdkConfig itself only exposes a way to check origins.

We can add other stuff later if/when we need it.

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• AWS SDK (ignoring whitespace)
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[ysaito1001]

The changes look great. I'll probably go through one more round to be sure.

[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• No codegen difference in the Client Test
• 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.

[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• No codegen difference in the Client Test
• 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.

[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• No codegen difference in the Client Test
• 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.

[ysaito1001]

Lastly, we probably need to have a changelog entry that revises or corrects release notes that we put out in https:/awslabs/aws-sdk-rust/releases/tag/release-2024-04-08 and the discussion topic, since the support is currently for endpoint url.

[Velfi]

Lastly, we probably need to have a changelog entry that revises or corrects release notes that we put out in https:/awslabs/aws-sdk-rust/releases/tag/release-2024-04-08 and the discussion topic, since the support is currently for endpoint url.

I updated the discussion. I think the Changelog entry is fine:

🎉 (#3493) Users may now set service-specific configuration in the environment. For more information, see #3537.