[Vertex AI] Add README for unit test mock files

[andrewheard]

Added a README.md with instructions for running the update_vertexai_responses.sh script. This file was also added as a resource in the FirebaseVertexAIUnit testTarget ensuring that a Bundle.module will always be generated by Swift Package Manager. This prevents the following build error if the script has not yet been run:

#no-changelog

[tanzimfh]

LGTM, thanks!

Maybe the text in README.md#firebase-vertex-ai should be replaced with a link to this README?

And should we add the script to scripts/README.md as well?

[andrewheard]

Maybe the text in README.md#firebase-vertex-ai should be replaced with a link to this README?

Agreed. Also I think people are more likely to land on the README in the top-level FirebaseVertexAI directory so I've moved this new content into there. I updated the README in the Unit subdirectory to link to it as well -- easier to maintain if it's all in one place. I should also add a section for the integration tests but will leave that for a follow-up PR.

And should we add the script to scripts/README.md as well?

Good catch, @tanzimfh! Added one there.