Task: Migrate use of `JacksonFactory` to `GsonFactory` in Google Cloud Sources

[Capstan]

https://github.com/cloudendpoints/endpoints-java/blob/master/endpoints-framework/src/main/java/com/google/api/server/spi/Client.java

This has already been patched in the monorepo, as security has advised us to prefer Gson over Jackson due to a history of security concerns.