refactor: Hides configuration from public interface and make isBuilt final

[gabrielgarciagava]

TL;DR This PR achieves what is described in this link: https://amplitude.com/docs/sdks/analytics/flutter/flutter-sdk-4-0-migration-guide#configuration

Configuration is immutable after you pass it to Amplitude.

Description

The purpose of these changes is because having both configuration and isBuilt publicly mutable is misleading, since changing those attributes will have no impact.

For example, currently it is possible to initialize Amplitude

final amplitude = Amplitude(Configuration(apiKey: "xxx"));

And then write

amplitude.configuration.flushQueueSize = 1;

Thinking this would lead to a on-the-fly reconfiguration of amplitude, which is not true.

Similarly but less important, I made isBuilt final since previosuly it would be possible to modify it, which makes no sense.

[gabrielgarciagava]

This removes a field from the public interface, which means it is a breaking change.
However, using this field is completely useless. If someone is using it, it is probably just a bug.

So my question is if this should create a new major release or not.

[Mercy811]

Hi @gabrielgarciagava, thanks for bringing this up. It makes sense to me that it's kind of misleading. We will make isBuilt final but will keep configuration there to avoid breaking changes.

[gabrielgarciagava]

Of course. But please consider hiding it in a next major release. Or at least making the field final, such as all configuration fields as well.

This way a user of the library will never be able to modify the configuration anymore after amplitude is initialized, which makes sense since it is not supported.

[gabrielgarciagava]

Just one use case to show you the motivation behind it.

When using version 3 we used to have the following code

final amplitude = Amplitude();
...
amplitude.enableCoppaControl();

When moving to version 4, someone just followed the naive substitution, which was allowed by the library interface

final amplitude = Amplitude(Configuration(apiKey: ...));
...
amplitude.configuration.enableCoppaControl = true;

Which of course is now wrong, but one needs to read the library implementation to realize it.

[chungdaniel]

As discussed, if you could revert the changes to hiding Configuration for now and leave in making isBuilt final I'd be happy to approve this and get this merged!