Add provider_name getter in Credentials impl

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### Describe the feature

Add a new `provider_name` method that returns the provider name of a `Credentials` instance.

### Use Case

It would be useful to be able to get the provider name of the credentials directly rather than hardcoding it. For example, in the case of a custom credentials provider building a new `Credentials` from an existing one.

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [x] 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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Locate the Credentials implementation and inspect its existing public accessors and related tests. Add the provider_name getter with behavior consistent with the current provider representation, then run the relevant credentials tests and confirm callers can retrieve the provider name without hardcoding it.

Written by the indexing model from the issue text.