Add generated Lex Runtime V2 client with integration tests

[Alan4506]

Description of changes:

This PR adds support for the Lex Runtime V2 service by adding the latest Smithy model and code-generating a new client package (aws-sdk-lex-runtime-v2). It also adds integration tests covering the following patterns:

• Non-streaming operation using recognize_text
• Bidirectional streaming operation using start_conversation (TEXT mode)

A setup script at tests/setup_resources.py creates the required AWS resources (IAM role, Lex V2 bot, en_US locale with a Greeting intent and closing response).

This branch will be rebased once the release automation publishes the client.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[SamRemis]

Left a few comments - Antonio was very thorough and got most of them.

To make sure I understand - are we expecting this to be the workflow:

1. A different PR adds the changelog entry and the models
2. We release the client via internal infrastructure
3. After confirming that this PR contains only the changes for the integ tests, we merge this PR so these get run for future releases automatically

Is that all correct?

[Alan4506]

Left a few comments - Antonio was very thorough and got most of them.

To make sure I understand - are we expecting this to be the workflow:

1. A different PR adds the changelog entry and the models
2. We release the client via internal infrastructure
3. After confirming that this PR contains only the changes for the integ tests, we merge this PR so these get run for future releases automatically

Is that all correct?

1. A different PR will be made to add the model, but as the comment says, the changelog entry might be handled by the release automation, so we won't add changelog entry in that different PR.
2. Yes, we will release the client via internal infrastructure
3. The PR may not only contain only the changes for the integ tests. After the client is released, in @jonathan343's opinion, there shouldn't be much difference between the this PR and upstream, except the integration tests, so the merge should be fine. But worst case if the merge conflict does happen, we can update this PR to only have the integ tests, or just create a new PR to only have the integ tests. But for now I'll just regenerate the client using the latest release to make sure all dependencies are up-to-date for testing, so that it is easier for reviewers to review the tests.

[SamRemis]

[Non-blocking]

Since the idea behind this is to have a single tag that can be applied to as many integ test related resources as possible, this would be better suited for smithy-aws-core and we can import it.

Since that would require a PR to smithy-aws-core and a publish, can we just make a backlog item for that and come back?