[Firestore] Re-export functions from '@google-cloud/firestore/pipelines'

[kevinthecheung]

Building queries with the pipelines API requires builder functions like and, field, etc from @google-cloud/firestore/pipelines. e.g.:

db.pipeline().collection('col').where(field('country').equal('USA'));

Should we re-export them?

[lahirumaramba]

Good call! I think we should re-export them here

firebase-admin-node/src/firestore/index.ts

Line 28 in c3de9df

export {