feat(go/genkit): Add vectorsearch vertex ai plugin

[rutujaD1999]

Description:

• Add new plugin for vectorsearch
• As part of the plugin the following functionalities have been implemented:
  • Added Init plugin
  • Added retriever functionality
    • This uses a document retriever function which is taken as input.
  • Added Index function for sample code. This can be customized by users to have their own index functions.
  • Added endpoints (upsert datapoints to index, find neighbors of a query) to insert and fetch documents depending on the index parameters
• Added retriever and indexer functions for Bigquery and Firestore - The indexer can be customized by the users to use their own.
• Added unit tests for vectorsearch functionalities
• Added sample code for bigquery and firestore docstores.

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)