HTTP client: Simplify HTTP file uploads and submitting forms

No implementation files, tests, or entry points are named. Start by evaluating the client API options described in the issue, including the server-side references (#252, #220, #226) and experimental reactphp/filesystem; done means an agreed API for forms and file uploads with a viable non-blocking filesystem approach.

This library should provide better support for HTTP file uploads and submitting forms. Right now, both are supported just fine, but require manually constructing the outgoing HTTP request body. This is relatively easy for simple forms (see https://github.com/reactphp/http#post), but more challenging for forms with file uploads.

Handling file uploads is already supported on the server side (see #252, #220, #226 and others), so adding better support for this also on the client side makes a lot of sense. Implementing this is non-trivial, but doable. The major challenge right now would be evaluating possible API options and once settled, sponsoring/funding for this feature.

On top of this, we would have to consider possible API options for adding file uploads. In particular, non-blocking filesystem access remains challenging (fstat + fopen + fread), see also experimental reactphp/filesystem.

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