feat: updated JwtClaims.Builder methods to public (#396)

Made this public so that the user could create JwtClaims instance.

Author: rahulKQL

oauth2_http/java/com/google/auth/oauth2/JwtClaims.java

@@ -110,15 +110,15 @@ public boolean isComplete() {
   }
 
   @AutoValue.Builder
-  abstract static class Builder {
-    abstract Builder setAudience(String audience);
+  public abstract static class Builder {
+    public abstract Builder setAudience(String audience);
 
-    abstract Builder setIssuer(String issuer);
+    public abstract Builder setIssuer(String issuer);
 
-    abstract Builder setSubject(String subject);
+    public abstract Builder setSubject(String subject);
 
-    abstract Builder setAdditionalClaims(Map<String, String> additionalClaims);
+    public abstract Builder setAdditionalClaims(Map<String, String> additionalClaims);
 
-    abstract JwtClaims build();
+    public abstract JwtClaims build();
   }
 }