Make HTTPClient internal

[pinarol]

Closes part of #449

Description

Making HTTPClient internal since a custom HTTPClient implementation will not work properly anyway. This is mostly because of how we handle and map errors. There's still a way to mock the URLSession via URLSessionProtocol.

Testing Steps

Smoke test:

SwiftUI demo app > Profile editor with oauth UIKit demo app > Profile cards

[wpmobilebot]

📲 You can test the changes from this Pull Request in Gravatar SwiftUI Prototype Build by scanning the QR code below to install the corresponding build.

	App Name	Gravatar SwiftUI Prototype Build
	Build Number	1456
	Version	1.0
	Bundle ID	com.automattic.gravatar-sdk-demo-swiftui.prototype-build
	Commit	2c2062a22cbda62284d2cda7a9101fbea17d5dd7
	App Center Build	Gravatar SDK Demo - SwiftUI #245

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Gravatar UIKit Prototype Build by scanning the QR code below to install the corresponding build.

	App Name	Gravatar UIKit Prototype Build
	Build Number	1456
	Version	1.0
	Bundle ID	com.automattic.gravatar-sdk-demo-uikit.prototype-build
	Commit	2c2062a22cbda62284d2cda7a9101fbea17d5dd7
	App Center Build	Gravatar SDK Demo - UIKit #245

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.