Add option for different ScrollPosition behavior on keyboard appearance

[lcoderre]

Hello 👋

Context
The default tableView.scrollTo with a ScrollPosition.none should animate in a way that the cell is fully visible. It works in most cases, but it seems to fail (scrolls out of view bounds) in the case of a cell preceding a very long footer. Most of the blame here is on Apple side, as I'll show below, however this change request could easily allow to bypass the issue.

Example/Experiment
In this commit (lcoderre@13c5d65) I modified the examples to add a very long Lorem Ipsum footer to a section, and also demonstrated using a pure UITableViewController with scrolling on every cell-click.

👀 - You might notice I recorded this on iPhone 11 pro simulator, but I also repeated the test on iPhone 8, 6s. Same Same.

👀 - Even a pure UITableViewController scrolls out of view for that last cell. And on the Eureka's Native Event UI, it acts a bit differently, but buggy all the same. It is worth noting that Eureka's scrolling behavior shown in the following gif isn't exactly the same as we're observing in our app using Eureka. In our case, it's more like Apple's version 🤷‍♂️ To be honest, I haven't dug in detail to find the reason.

See scrolling using .none

See now using .top (👀 no bug)

Proposed solution
The proposed solution has the advantage of being "additive" and not "disruptive". I.e. it won't break anyone's code, but will allow some of us to bypass a nasty problem :)

⚠️ I have not considered extending the pattern to other places where there is scrolling, since I did not know if there was a need. Whereas in the context of keyboard showing up, making sure the cell with textfield was shown was my priority.

[mats-claassen]

Thanks for the investigation. Adding a parameter like this is always easy and just adds functionality. I think it might be desired at some point to define the scroll position on a per row basis but let's leave that for now