STS AssumeRoleWithWebIdentity example

### Describe the feature

add a STS AssumeRoleWithWebIdentity example

### Use Case

Returns a set of temporary security credentials for users who have been authenticated in a mobile or web application with a web identity provider.

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### A note for the community

### Community Note
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No target file or test is named. Start by locating existing AWS STS examples and the AssumeRoleWithWebIdentity entry point, then determine the repository's example conventions; done means the requested STS web-identity example is present and follows those conventions.

Written by the indexing model from the issue text.