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

[alonsopec89]

Add context caching to vertexai
#1443

How to test:

func main() {
    fmt.Println("hello world!!!!")
    ctx := context.Background()
    if err := vertexai.Init(ctx, nil); err != nil {
        log.Fatal(err)
    }
    m := vertexai.Model("gemini-1.5-flash-002")
    if m == nil {
        log.Fatal(errors.New("vertexai 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)

[alonsopec89]

context caching is working only with stable Gemini model versions using number prefix like https://cloud.google.com/vertex-ai/generative-ai/docs/learn/model-versions#stable-versions-available

[apascal07]

We don't want to get rid of the plain gemini-1.5-flash. @hugoaguirre is working on adding model version support per model family, please work with him on how combine the two efforts.

[apascal07]

Please add tests for the caching business logic and a sample where this is used (by setting the cache TTL, which enables context caching). See #1297 for test coverage.

[apascal07]

Go style guide uses tabs (as 4 spaces) instead of 2 spaces. Please update here and in the other PR.

[apascal07]

We don't want to get rid of the plain gemini-1.5-flash. @hugoaguirre is working on adding model version support per model family, please work with him on how combine the two efforts.