add validation to ensure ImpersonatedCredential can only be SA or User Credentials

[zhumin8]

This line in javadoc specifies ImpersonatedCredential can only be SA or User Credentials in this create() method, but it is not enforced.

google-auth-library-java/oauth2_http/java/com/google/auth/oauth2/ImpersonatedCredentials.java

Line 120 in a8d466f

* should be either a user account credential or a service account credential.

For context: #1566 (comment)