Allow AwsCredentials/ExternalAccountCredentials to take in customized environmentProvider

[hlf0319]

Is your feature request related to a problem? Please describe. I'm always frustrated when I want to use my own temporary (AWS) credentials to fetch jwt. The only way to pass in my own environmentProvider is through the System env right now. But I want to rotate the info inside environmentProvider over the time, setting System env is not a great choice. EnvironmentProvider interface provided by the lib is package level so I could not implement my own and pass in.

Describe the solution you'd like Expose EnvironmentProvider and allow me to pass in my own implementation of it to AwsCredentials/ExternalAccountCredentials.