feature(go/plugins/vertexai): create context caching for vertexai

[alonsopec89]

Try the sample in go/samples/cache/main.go
don't forget to set the env vars

os.Setenv("GCLOUD_PROJECT", "some gproject number")
os.Setenv("GOOGLE_GENAI_API_KEY", "your genai api key")

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)