[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.

找到 AwsCredentials、ExternalAccountCredentials 和包级别的 EnvironmentProvider 接口，然后了解凭据目前如何获取环境提供程序。调用方能够向两种凭据类型提供自定义 EnvironmentProvider，并且相关的现有测试覆盖这一注入路径后，工作即告完成。

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