Add VertexAI extension

[Lash-L]

Describe Your Changes

• This adds a new extension for VertexAI
• I am currently facing a glitch where the models are disabled on each close and reopen until I open up the settings page(Don't have to change anything or even go to the VertexAI settings page), but I'm hoping this is something a dev can walk me through fixing - as there is likely some check happening that determines if a model should be enabled or not.

Relates to:
#3786

Self Checklist

• Added relevant comments, esp in complex areas
• Updated docs (for bug fixes / features)
• Created issues for follow-up changes or refactoring needed