Properly encode literal `+` characters in `URLQueryItem`s

[andrewdmontgomery]

Closes #404

This PR is meant to replace #406. See that PR for a detailed discussion of why this PR exists.

Description

This introduces a method for adding URLQueryItem's to a URLComponents object, and encapsulates the encoding of + literal characters.

This is a simplified approach. See #406 for a more comprehensive, and more fragile, solution.

Testing Steps

See #404 for details

• [ ] All unit tests should pass
• [ ] Smoke test avatars
• [ ] Try Profile editor with oauth in the SwiftUI demo app. Enter an email address that contains a + character, and observe that the auto-populated email address in the OAuth web view contains that character (and not a space).

[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	1268
	Version	1.0
	Bundle ID	com.automattic.gravatar-sdk-demo-uikit.prototype-build
	Commit	2ea22c711dc5fafddc4bd06103f76e5925c17032
	App Center Build	Gravatar SDK Demo - UIKit #139

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[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	1268
	Version	1.0
	Bundle ID	com.automattic.gravatar-sdk-demo-swiftui.prototype-build
	Commit	2ea22c711dc5fafddc4bd06103f76e5925c17032
	App Center Build	Gravatar SDK Demo - SwiftUI #138

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

[andrewdmontgomery]

If we agree that this is a safer approach, I will open a PR to adjust our fix in PocketCasts: https://github.com/Automattic/pocket-casts-ios/pull/1770

As it is now, that fix should not crash. But that approach encodes too much, and will make it possible for a future developer to introduce a change that will result in runtime fatalErrors.

[andrewdmontgomery]

https://github.com/Automattic/Gravatar-SDK-iOS/pull/406#issuecomment-2356457161

Of all the characters that are allowed in a Query String, there is only one character whose meaning is defined by the implementation (us) – the + character's meaning is up to us. It is valid to use it as an encoded space. It is also valid to use it as a literal + character.

It would therefore be a bug for Apple to automatically encode the + character.

It is up to us to decide whether the + character is literally, and if so, to encode it.

Since this is the only character that we need to handle manually (based on our own needs), it is simpler and safer to update the percentEncodedQuery property, to encode + characters, than to bypass the built-in encoding and implement our own.