evaluate refactor builder patterns so that setters do not need to be override in child classes.

[zhumin8]

see this comment for context: #1528 (comment)

builders are not created with generic types across this repo e.g.

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

Line 486 in f154edb

public static class Builder extends OAuth2Credentials.Builder {

This means to allow customer code to use a fluent builder API without casting, setters in parent classes need to be overriden in child classes, even without adding specific logics.

Note that adding generics to existing classes may also produce warning regarding raw use of generic types.