Add a common interface for resource field enums

[mziccard]

We should consider creating an interface for all field enums (e.g. DatasetField in gcloud-java-bigquery andBlobField in gcloud-java-storage).

interface ResourceField {
  String selector();
}

Selector's creation code (at least part of it) should be moved to a common static method.

[aozarov]

For background see this and this.

[mziccard]

Fixed with #807

[aozarov]

@mziccard I just wonder if we should keep this one open until you also apply the fix to compute?

[mziccard]

Keeping this open until it's fixed in compute