Firestore: Support passing a slice of strings when querying `firestore.DocumentID IN ["docId, "docId2"]`

**Client**

FireStore

**Environment**

Win10

**Go Environment**
go version: go1.16.3 windows/amd64

**Code**

e.g.
```go
iterator := someCollection.Where(firestore.DocumentID, "in", someSlice).Documents(context.Background())
```

**Expected behavior**

Iterator contains all the snapshots of documents of which their ID is present in the `someSlice` slice

**Actual behavior**

Error is thrown: `rpc error: code = InvalidArgument desc = __key__ filter value must be a Key`

**Additional context**

This should work following the answers on [this](https://stackoverflow.com/questions/47876754/query-firestore-database-for-document-id) thread