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

[bhshkh]

Checklist

• I have followed Contributing Guidelines from CONTRIBUTING.MD
• Tests pass: go test -v ./.. (see Testing)
• Code formatted: gofmt (see Formatting)
• 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:
googleapis/google-cloud-go#10802
#4358
googleapis/google-cloud-go#10597

[snippet-bot]

Here is the summary of changes.

You are about to add 3 region tags.

• firestore/vector_search_distance_threshold.go:17, tag firestore_vector_search_basic
• firestore/vector_search_result_field.go:17, tag firestore_vector_search_distance_result_field
• firestore/vector_search_result_field_masked.go:17, tag firestore_vector_search_distance_result_field

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[telpirion]

Only small nits--address as you see fit. I'll merge this PR after your next comment. Thanks!