feat(generativeai): Add compute token feature

[msampathkumar]

Description

Fixes b358102024

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Checklist

• I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• README is updated to include all relevant information
• Tests pass: nox -s py-3.9 (see Test Environment Setup)
• Lint pass: nox -s lint (see Test Environment Setup)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[msampathkumar]

Update: When I used SDK @ google-cloud-aiplatform[tokenization] @ git+https://github.com/googleapis/python-aiplatform.git the following errors don't come up. I will wait for the pkg to be released.

TypeError: Tokenizer.count_tokens() got an unexpected keyword argument 'system_instruction'

AttributeError: 'ComputeTokensResult' object has no attribute 'tokens_info'