Prevent stacking of toast messages

[pinarol]

Closes #457

Description

Upon a request from our designer, i implemented a mechanism to prevent the toasts from stacking. If the new toast has the same message, the previous one will be dismissed first.

Testing Steps

SwiftUI demo app > Profile editor with oauth Switch between different avatars and observe the you see only 1 success toast at a time. Disconnect from internet, try the same thing and observe it for the fail message as well

[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	1391
	Version	1.0
	Bundle ID	com.automattic.gravatar-sdk-demo-swiftui.prototype-build
	Commit	ec73a99b66b99a61d1e10c4d04bedb23cab59e63
	App Center Build	Gravatar SDK Demo - SwiftUI #211

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	1391
	Version	1.0
	Bundle ID	com.automattic.gravatar-sdk-demo-uikit.prototype-build
	Commit	ec73a99b66b99a61d1e10c4d04bedb23cab59e63
	App Center Build	Gravatar SDK Demo - UIKit #211

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