Add ability to make simple requests and parse them for a functional prototype

[oguzkocer]

This PR connects the dots in the current implementation to be able to make simple requests and parse them for a functional prototype. A lot of how that's implemented is through temporary means as the design for several of the components are not fully fleshed out yet. So, the implementation has a lot of TODOs to get this to a more production ready state.

Considering that this project as a whole is in its prototype stage, we need parts of it to be throwaway code to get it to a stage where we can test things out. Scaffolding can only do so much for us and we needed something like this to test its integration with native layers.

There is a good chance this prototype will become a proper project and that's why I wanted to leave a lot of TODOs so we don't end up using throwaway code in production. I think I'll get to address some of these TODOs before this goes out of prototype stage, but some of it I intend to keep, because it's just not worth polishing it until we address the more important parts.

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As for the implementation, I found the design we landed on to be pleasant to work in. I found some parts to be a little bit annoying - such as not being able to implement From trait in wp_networking crate for types in the wp_api crate. However, the frustrations were generally minor and always had straightforward replacements.

I really like how simple we can keep WPNetworking implementation and how it can be easily replaced without knowing any details about the requests. I also like that the authentication headers can be built into the WPNetworkRequest from wp_networking crate and not require those details to be directly exposed to a custom networking layer.

I am not quite sure about error handling. I have a decent understanding of the possibilities in Rust ecosystem (although it can be a bit of a hard topic to get right even in pure Rust crates) and uniffi bindings. However, I don't have a clear picture of how exactly we'll structure our errors and what will be exposed in each layer. I don't think this is a problem that we'll be able to figure as part of this prototype. So, we might do a throwaway implementation just to test how it could possibly integrate into native layers.

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If you'd like to test this using the wp_cli, you need to add a test_credentials file at the root of the project where the first line is the site url and second line is the base 64 encoded authentication token of {username}:{application_password}:

https://foo.bar
{authentication_token}