feat: cleaning up http client interface and adding the ability to use custom http clients

[izaaz]

Summary

1. Moving http related classes into a package
2. Refactoring the http client class to expose a single method called upload
3. Creating an interface
4. Testing it out with an OkHttp custom client

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• Does your PR have a breaking change?:

[igor-amp]

lgtm

[github-actions]

🎉 This PR is included in version 1.20.0 🎉

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• v1.20.0

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[cbeyls]

How are customers supposed to implement a custom API client properly without having access to getServerUrl() or minIdLength from the internal configuration? Hardcoding these values is definitely not a good option.

[izaaz]

The initial plan was to make our http client an open class which can be extended. But we were running into a circular dependency.

The http client needs a configuration to be initialized and the custom http client needed to be passed into the configuration so that the Amplitude client can be configured to use a custom http client.

Since the HttpClient is an interface, you can define your class that takes in the Amplitude configuration to fetch all the properties from the configuration directly.

For example

class CustomClient() : HttpClientInterface {
    fun initialize(configuration: Configuration) {
        ....
    }

    // override upload here 
}

And in init

val customClient = CustomClient()
val config = Configuration(...)
customClient.initialize(config)
val amplitude = Amplitude(configuration)

I can update the example to show this and also open to suggestions.

[izaaz]

Something like this: #240