feat(firestore): Distance result field and distance threshold in vector search

[bhshkh]

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

[bhshkh]

To be merged after: https://github.com/googleapis/google-cloud-go/pull/10802 https://github.com/GoogleCloudPlatform/golang-samples/pull/4358 https://github.com/googleapis/google-cloud-go/pull/10597

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 3 region tags.

- [firestore/vector_search_distance_threshold.go:17](https://github.com/bhshkh/golang-samples/blob/5bb37dce6d9e9be329cd54e1606355677a59c479/firestore/vector_search_distance_threshold.go#L17), tag `firestore_vector_search_basic` - [firestore/vector_search_result_field.go:17](https://github.com/bhshkh/golang-samples/blob/5bb37dce6d9e9be329cd54e1606355677a59c479/firestore/vector_search_result_field.go#L17), tag `firestore_vector_search_distance_result_field` - [firestore/vector_search_result_field_masked.go:17](https://github.com/bhshkh/golang-samples/blob/5bb37dce6d9e9be329cd54e1606355677a59c479/firestore/vector_search_result_field_masked.go#L17), tag `firestore_vector_search_distance_result_field`

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