This change adds support for requesting temporary object storage
credentials using the STS API. Some breaking changes are introduced to
enable this support:
- `Credentials` type has been removed. Use the `CredentialValue` type
instead. Corresponding to this the type signature for `setCreds` has
changed, though the functionality is the same.
- The type alias `Provider` has been renamed to `CredentialLoader` to
avoid naming confusion.
This exports a new module for retrieving STS based credentials, however
they are not yet convenient to use in the library - the session token
needs to be included as a custom header and may not be possible with all
APIs.
