feat(stepfunctions): Support calling TestState API from Workflow Studio

[anthonyting]

Problem

The Workflow Studio webview currently does not allow for calling the TestState API. This API is used for testing individual states in isolation, and helps with debugging when constructing a state machine. It is available in the console version of Workflow Studio.

Solution

Adding support for calling APIs from the webview using message passing. This is the added flow:

1. The webview sends a message to the extension to call sfn:TestState or iam:ListRoles
2. The extension performs the API call using its credentials and default credential region
3. The extension sends the response as a message to the webview

Note: this PR is dependent on this PR being merged first since it requires an aws-sdk version update.

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[github-actions]

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