Provide builder for ProjectListOption.filter()

[ajkannan]

Then the user doesn't have to enter large strings that combine multiple filters themselves.

[danoscarmike]

This has been added to our feature backlog: https://github.com/GoogleCloudPlatform/google-cloud-java/wiki/Feature-backlog. This issue will be closed but is linked in the backlog and can continue to be used for comment and discussion.