feat(go/plugins/googleai): add context caching to googleai

[alonsopec89]

Add context caching to vertexai
#1443

How to test:

func main() {
    fmt.Println("hello world!!!!")
    ctx := context.Background()
   err := googleai.Init(ctx, &googleai.Config{APIKey: os.Getenv("GOOGLE_GENAI_API_KEY")})
    if err != nil {
        log.Fatal(err)
    }
    m := googleai.Model("gemini-1.5-flash")
    if m == nil {
        log.Fatal(errors.New("googleai init failed"))
    }

    strrr, err := ai.GenerateText(ctx, m, ai.WithTextPrompt("Tell me a joke about golang developers."))
    if err != nil {
        fmt.Println(err)
    }
    fmt.Println(strrr)

    strrr, err = ai.GenerateText(ctx, m, ai.WithTextPrompt("Tell me a joke about golang developers. "))
    if err != nil {
        fmt.Println(err)
    }
    fmt.Println(strrr)
}

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)