Make helper for loading credentials providers from environment variables

[UgnineSirdis]

There is an algorithm for loading credentials providers of different types from environment variables described in YDB doc: https://ydb.tech/docs/en/reference/ydb-sdk/auth#env

There is some implementation of it, but it is located in Yandex.Cloud specific library: https://github.com/ydb-platform/ydb-java-yc/blob/master/provider/src/main/java/tech/ydb/auth/iam/CloudAuthHelper.java

If user doesn't use this library, but wants to load credentials provider from env, they don't have this ability now.