Add user configurable embedding model and quota (LlamaIndex on Vertex AI for RAG)

Description

Fixes #

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[ ] 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
[x] Please merge this PR for me once it is approved