Add InsertAllRequest, InsertAllResponse and model classes for table data

[mziccard]

This PR will probably require some discussion :) I am eager to receive any feedback.
It adds:

• InsertAllRequest and InsertAllResponse to insert data using TableData: insertAll.
• TableRow and FieldValue to hold the results of TableData: list and query

Remarks:

• We discussed with @aozarov about adding more information to FieldValue.Kind as REPEATED_OF_PRIMITIVE and REPEATED_OF_RECORD but I realized that an empty repeated field is returned as an empty array and there's no way to say whether it is of primitive or record type.

[mziccard]

Thanks @ajkannan for the pass. I fixed the comments in place.

[mziccard]

Should we make all value getters in FieldValue throw the same exception? Right now they either throw ClassCastException, IllegalStateException, NumberFormatException or NullPointerException depending on the type of conversion being asked. What do you think?