feat(firestore): Vector search

[bhshkh]

Description

Adding vector search samples This is similar to Python and Node in https://firebase.google.com/docs/firestore/vector-search

Checklist

• [x] I have followed Contributing Guidelines from CONTRIBUTING.MD
• [x] Tests pass: go test -v ./.. (see Testing)
• [x] Code formatted: gofmt (see Formatting)
• [x] Vetting pass: go vet (see Formatting)
• [ ] 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

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 3 region tags.

- [firestore/vector_search_basic.go:17](https://github.com/bhshkh/golang-samples/blob/423e3436088e8b983d17634321729ee106dba3d0/firestore/vector_search_basic.go#L17), tag `firestore_vector_search_basic` - [firestore/vector_search_prefilter.go:17](https://github.com/bhshkh/golang-samples/blob/423e3436088e8b983d17634321729ee106dba3d0/firestore/vector_search_prefilter.go#L17), tag `firestore_vector_search_prefilter` - [firestore/vector_store.go:17](https://github.com/bhshkh/golang-samples/blob/423e3436088e8b983d17634321729ee106dba3d0/firestore/vector_store.go#L17), tag `firestore_store_vectors`

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[telpirion]

I am SO excited to see how I can do vector search with my favorite database product.