ARROW-2604: [Java] Add convenience method to VarCharVector to set Text

[BryanCutler]

This adds a convenience method to easily set values in VarCharVector using the friendly type Text. This allows the user to set values without having to think about the correct encoding. Text objects can be constructed from a String using the constructor Text(String string).

Extended existing test to set/get a VarCharVector using Text objects.

[BryanCutler]

I had also wondered why there was no method to set a String directly in VarCharVector but after looking into it, it seems like the "friendly type" is Text - this is the type of object that the vector returns in getObject. It seems like maybe the Text class is being encouraged over String, so I only added the ability to set a Text and not String directly, although it is easy to create Text from String. I'm neutral on whether or not to also add set methods for a String, but maybe it would make the API too crowded and be confusing to the user. What are your thoughts @siddharthteotia @icexelloss @xhochy ?

btw, it seems hadoop users are familiar with using Text over String..

[icexelloss]

LGTM! Thanks!

[icexelloss]

I am curious about the choice of using Text over String too, maybe @jacques-n can shed some lights?

[BryanCutler]

I'll commit this later today if no more comments. We can discuss further if there is still interest in adding an api directly for setting Strings.

[siddharthteotia]

+1

[BryanCutler]

Thanks @icexelloss and @siddharthteotia ! merged to master