fix(firestore-bigquery-export) use modules instead of namespaces

[yutak23]

This is a PR to correct the error in the following issue.
#1533

After checking where the error is occurring, it appears that there is a problem with the require syntax that is importing the module.
I think that the correct code should be as follows (typescripts build to javascripts).

correct:

const firestore = require("firebase-admin/firestore");

In other words, I think the problem is using the global admin namespace, as in the following code.
https://github.com/firebase/extensions/blob/firestore-bigquery-export-v0.1.31/firestore-bigquery-export/firestore-bigquery-change-tracker/src/bigquery/index.ts#L18

This should be redefined as follows (I am aware that it says so in Use modules instead of namespaces).

import { DocumentReference } from 'firebase-admin/firestore';
...
    serializeData(eventData) {
        ...
                if (property.constructor.name === DocumentReference.name) {
                    this.update(property.path);
                }
            }
        });
        return data;
    }

[yutak23]

This is my first time drafting an issue and creating a PR for OSS.
Cloud you let me know if there is anything I have missed.

[yutak23]

Fixed with reference to the following issue.

firebase/firebase-admin-node#1465

[yutak23]

and i confirm npm run test result is ALL Green.

[cabljac]

lgtm!

[cabljac]

CI isn't running for some reason, closing in favour of #1561