feat(firestore): Query explain and explain analyze

[bhshkh]

Description

Adding Firestore query explain and explain analyze samples. The build will pass once 1.17.0 has been released. I will merge this PR once https://github.com/googleapis/google-cloud-go/pull/10597 has been merged. This has been tested locally by pointing cloud.google.com/go/firestore to local directory

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 4 region tags.

- [firestore/aggregate_query_explain.go:17](https://github.com/bhshkh/golang-samples/blob/55c855d4ac5736fd80843db1da6e42701d9b63da/firestore/aggregate_query_explain.go#L17), tag `firestore_query_explain_aggregation` - [firestore/aggregate_query_explain_analyze.go:17](https://github.com/bhshkh/golang-samples/blob/55c855d4ac5736fd80843db1da6e42701d9b63da/firestore/aggregate_query_explain_analyze.go#L17), tag `firestore_query_explain_analyze_aggregation` - [firestore/query_explain.go:17](https://github.com/bhshkh/golang-samples/blob/55c855d4ac5736fd80843db1da6e42701d9b63da/firestore/query_explain.go#L17), tag `firestore_query_explain_entity` - [firestore/query_explain_analyze.go:17](https://github.com/bhshkh/golang-samples/blob/55c855d4ac5736fd80843db1da6e42701d9b63da/firestore/query_explain_analyze.go#L17), tag `firestore_query_explain_analyze_entity`

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[telpirion]

Please be sure to address the errors from vet.