Utilize targetprocess v2 api for GET requests

[lucasreed]

After looking closer at the docs, I think it would be prudent to use the v2 api for GET requests. As documented here there are some powerful filters that will be very useful that are not available in the v1 api.

The data you get back from some of these more advanced features should allow for a single API call that would have required multiple in the v1 api.

[lucasreed]

As an example, in the v1 API, there is no good way to get the assigned person for a given UserStory and would likely require either:
a) a lot of changes to our structs to get data (testing this idea, I couldn't get it to work though)
b) multiple api calls to finally get to an Assignable

With the v2 api, I could do:

err := tpClient.Get(&o, "Assignments", nil, tp.Where("Assignable.EntityType.Name eq 'UserStory'"), tp.Where("GeneralUser.FirstName eq 'Luke'"))

Bonus: the Where() filter will still work as written, but we can add a few more to utilize the other ones mentioned in the docs.