[4.x] Add a default cURL HTTP client

[cleptric]

Work in Progress

This adds a cURL-based HTTP client and simplifies the setup when using a custom Transport or HttpClient.
As mentioned in #1586, this removes the /store endpoint entirely. All events are now sent to the /envelope endpoint.

[Jean85]

As I've already said in #1586 (comment), I'm against writing our own HTTP client but, since I recognize I don't have a final say in this, I'm still giving my review.

I would at least suggest leveraging PSR-18 (HTTP client) and so PSR-7 (request/response) with it, but we could ditch PSR-17 (factories) to keep it simple.

[ste93cry]

I agree with everything already said by @Jean85: first of all, I don’t understand the reason to have our own HttpClientInterface instead of just using the PSR-18 one. There is no benefit I can think of, it would be just another layer that needs to be implemented by the end user. Second, and more importantly, I don’t understand why we are reimplementing a full cURL client, which we have to maintain and for which there is no benefit because it’s a lot simpler and less customizable than any other out of here. Moreover, what’s happening here is that we are reinventing the wheel: I can understand that this SDK has to ship with a default client, even though I don’t agree, but at least choose one reliable client instead of reinventing the wheel. We had our own HTTP client in the past, and it was full of issues. Given that the SDK is integrated with both Laravel and Symfony, it would be wise to choose symfony/http-client which is common for both

[stayallive]

Overal this is starting to look great, mostly because it's a heck of a lot simpler.

[stayallive]

👍🚀

[cleptric]

Merging now to unblock other features. The remaining ToDos are documented in #1595