Add UUID to each pageview event

[wzieba]

Closes: #28

Description

This PR is a twin of #73 for pageview and heartbeat events. It adds a new, internal field lastPageviewUuid. See internal discussion about necessity of it: p1696417656257339-slack-C0533SEJ82H

Test

1. Install example app, open logcat
2. Tap on Track Url
3. Tap on Start Engagement
4. Wait 30 seconds until you see [Parsely] POST Data {"events": (...) log. Copy the JSON part.
5. Wait another 30 seconds until another [Parsely] POST Data {"events": (...) log, copy it as well.
6. Check the copied logs:

• there should be one "action": "pageview" event, rest of them should be "action": "heartbeat"
• every event from both logs should have "pvid" field with the same value

New payload

Click

[ParaskP7]

👋 @wzieba !

I have reviewed and tested this PR as per the instructions, everything works as expected, good job! 🌟

---

1. Thinking (🤔): Although I am "kind of" not a fan of having this new lastPageviewUuid state added and using it as such, depending that the clients would first trackPageview(...) and then startEngagement(...), just because it is in the docs, since I understand that this was discussed and agreed on over Slack, I am okay keeping it as is.
2. Warning (⚠️): Let's deal with this here and remove it before merging this PR to master.

[wzieba]

Let's deal with #73 (comment) here and remove it before merging this PR to master.

Thanks for being vigilant! Addressed in 61c3db3