feat(firestore): add timestamp & fieldvalue management

[mamillastre]

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Close #474, #443

Linked PR: #557

Usage example:

import { FirebaseFirestore, Timestamp, serverTimestamp } from "@capacitor-firebase/firestore";

FirebaseFirestore.setDocument({
    reference: "collection/doc",
    data: {
      timestamp: Timestamp.now(),
      fieldValue: serverTimestamp()
    },
 });

[mamillastre]

Hi @robingenz,

For now, this PR is a prototype of the Timestamp & FieldValue management on Web and Android.

Do you have some time to make early feedbacks on this before I continue the development?

Especially on these points:

• You advise me here to manage the data parsing from the native layer like the Bluetooth Low Energy Plugin. But I have to implement all the plugin methods, even those that are out of scope.
  Here, I use a Proxy. Tell me if you are ok with this solution.
• When using the plugin, the firebase dependency is mandatory for the Timestamp & FieldValue usage. But I think you prefer avoiding this.
  In this case, I see two possibilities:
  1. Copying the Timestamp code from the firebase js SDK into the plugin
  2. Set the Timestamp implementation in a separated index.js file to avoid to install the firebase dependency if the timestamp is never used

Thank you

[robingenz]

@mamillastre Thanks for the update. I'm currently on vacation and won't be back for another 2 weeks. I'll have a look at it as soon as i'm back.