[java-datastore] Make `fromPb` constructors public

**Is your feature request related to a problem? Please describe.**
I often run across situations where I have either a `com.google.datastore.v1.Key` or `com.google.datastore.v1.Entity` and would like to transform it into a `com.google.cloud.datastore.[Key|Entity]`. There currently exists `fromPb` constructors within this codebase, but they are not accessible outside of the `com.google.datastore` namespace.

**Describe the solution you'd like**
Making the `public`.

**Describe alternatives you've considered**
I currently have two interop packages of my own living in `com.google.datastore` that re-export these methods. It works but isn't great.