Make DocumentBuilder handlers into generic types

[morgs32]

So that we can type our functions better!

Description

Code sample

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[morgs32]

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[taeold]

Hey @morgs32. Thanks for taking the time to contribute.

While reviewing the code, I noticed that both DocumentSnapshot and QuerySnapshot types already default to DocumentData:

https://github.com/googleapis/nodejs-firestore/blob/28d645bd3e368abde592bfa2611de3378ca175a6/types/firestore.d.ts#L1121

https://github.com/googleapis/nodejs-firestore/blob/28d645bd3e368abde592bfa2611de3378ca175a6/types/firestore.d.ts#L1488

At least in VSCode, the typescript language server seems to make use of this type annotation:

I'm pretty new to Typescript, and I'm unsure what features are unlocked when we make the proposed changes. Do you mind helping me understand by sharing an example or two?

[morgs32]

This would enable something like this - in which I can easily type the snapshot of the onWrite. Let me know if this makes sense @taeold

export const onWriteUserFirestoreHandler: onWrite<IUser> = async (props) => {

  const {
    after,
    // before
  } = props

  const userAfter = after.data()

  if (!userAfter) {
    return
  }

  // const userId = userAfter.id;

}