Implement initial lazy caching credentials provider (#578)

* Implement initial lazy caching credentials provider

* Rename TimeProvider to TimeSource

* Move TimeSource to its own module

* Eliminate Inner layer and add expiry_mut to Credentials

* Move Cache to its own module and fix multithreading issue

* Add comments

* Make refresh_timeout unimplemented

* Combine Provider with LazyCachingCredentialsProvider

* CR feedback