[google-auth-library-java] Allow AwsCredentials/ExternalAccountCredentials to take in customized environmentProvider

**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.