[image-to-text pipeline] Add conditional text support + GIT
# What does this PR do?

The `ImageToText` pipeline can generate text given an image, but oftentimes one wants to make the model continue text given a prompt (like "a photo of"). This PR adds support for conditional text generation given an image.

It also adds support for GIT.

This PR fixes a part of #21110 and is based on #22423.

To do:

- [x] add support for Pix2Struct once design is approved

cc @younesbelkada 
_The documentation is not available anymore as the PR was closed or merged._<|||||>@Narsil thanks for your review, feel free to approve.<|||||>Apologies, will take this into account.