feat(vertexai): Context caching

[Deleplace]

Adapted from the Python samples for Context caching

Region tags:

generativeaionvertexai_gemini_create_context_cache]
generativeaionvertexai_gemini_get_context_cache
generativeaionvertexai_gemini_use_context_cache
generativeaionvertexai_gemini_update_context_cache
generativeaionvertexai_gemini_delete_context_cache

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 5 region tags.

- [vertexai/context-caching/contextcaching_create.go:18](https://github.com/Deleplace/fork-golang-samples/blob/144233d4dc81ab54d1bb8bda3813c020758e135b/vertexai/context-caching/contextcaching_create.go#L18), tag `generativeaionvertexai_gemini_create_context_cache` - [vertexai/context-caching/contextcaching_delete.go:18](https://github.com/Deleplace/fork-golang-samples/blob/144233d4dc81ab54d1bb8bda3813c020758e135b/vertexai/context-caching/contextcaching_delete.go#L18), tag `generativeaionvertexai_gemini_delete_context_cache` - [vertexai/context-caching/contextcaching_get.go:18](https://github.com/Deleplace/fork-golang-samples/blob/144233d4dc81ab54d1bb8bda3813c020758e135b/vertexai/context-caching/contextcaching_get.go#L18), tag `generativeaionvertexai_gemini_get_context_cache` - [vertexai/context-caching/contextcaching_update.go:18](https://github.com/Deleplace/fork-golang-samples/blob/144233d4dc81ab54d1bb8bda3813c020758e135b/vertexai/context-caching/contextcaching_update.go#L18), tag `generativeaionvertexai_gemini_update_context_cache` - [vertexai/context-caching/contextcaching_use.go:18](https://github.com/Deleplace/fork-golang-samples/blob/144233d4dc81ab54d1bb8bda3813c020758e135b/vertexai/context-caching/contextcaching_use.go#L18), tag `generativeaionvertexai_gemini_use_context_cache`

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[Deleplace]

Note that createContextCache returns a string. This is necessary for the test to refer to the cached content that it has just created.

[ymotongpoo]

@Deleplace do you mean this is port of these samples? (the link you mentioned is pointing at controlled generation) https://github.com/GoogleCloudPlatform/python-docs-samples/tree/621454ef68f7fbda02060d1a249acb83d5e490d6/generative_ai/context_caching

[Deleplace]

@Deleplace do you mean this is port of these samples? (the link you mentioned is pointing at controlled generation) https://github.com/GoogleCloudPlatform/python-docs-samples/tree/621454ef68f7fbda02060d1a249acb83d5e490d6/generative_ai/context_caching

Thanks! Indeed the link was wrong, I updated it at the top of the PR.