chore: add identify and base event

[Mercy811]

Description

• Add Identify and BaseEvent
• The old /lib/identify.dart is still there for dev reference, I will remove it after updating the main Amplitude interfaces like amplitude.identify().

User interfaces

final identify = Identify()
    ..set(property: "color", value: "green")
    ..setOnce(property: "property", value: "value")
    ..add(property: "property", value: "value");

// Event type is required
final event1 = BaseEvent(eventType: "button clicked");
// Can also pass other fields when creating a base event
final event2 = BaseEvent(eventType: "button clicked", eventProperties: {"button color": "green"});

Tests

identify

Some logs in identify are not covered. Didn't find an easy way to intercept logging stream or mock and replace the logger.

[yuhao900914]

@Mercy811
Maybe we can skip the log?
https://stackoverflow.com/a/23264375

[Mercy811]

Thanks @yuhao900914 I will add it in the next PR.

[Mercy811]

It's for C++ and doesn't work in Dart 😔

[yuhao900914]

Good to know. Thanks Xinyi.

[github-actions]

🎉 This PR is included in version 4.0.0-beta.1 🎉

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