Add to Builder methods to Datastore's classes

[mziccard]

Datastore's ProjectionEntity, IncompleteKey, Entity and Key have static builder methods:

public static Builder builder(ProjectionEntity copyFrom)
public static Builder builder(IncompleteKey copyFrom)
public static Builder builder(Entity copyFrom)
public static Builder builder(Key copyFrom)

In other modules we use a toBuilder() instance method to create builders from entity objects. Should we use toBuilder() also in Datastore? Or is there a reason to prefer static builder(ResourceType copyFrom)? \cc @ajkannan @aozarov

[ajkannan]

It would be nice to add toBuilder, but I encountered some issues with inheritance when I tried to add those methods (for keys). I briefly described the problem I ran into in #628.

[mziccard]

Now I see, it seems more complex than I expected. I am closing this for now. Will reopen in case somebody else feels like those methods should be added. Thanks @ajkannan.