Update cross platform vpn logic for v2 credentials

[iccub]

Description

We may (or may not) need to make changes to accomodate the SKU credentials v2

No changes required to use the new system; but we will be required to cache the subscriber credentials once we get them to avoid burning through all of the credentials for a given day. Unlike before, there are only a certain number of preallocated credentials for each day

[bsclifton]

Carried over from brave/brave-browser#26254

(notes removed; see requirements below)

[bsclifton]

Thanks @iccub for walking through the code with me over video chat 😄

For reference, we made the changes needed for Desktop and Android with brave/brave-core#15702

Here are the requirements we needed to satisfy:

1. When calling prepare_credential_presentation, we get SKU response as cookie; we need to save the expiration time also.
   

   brave-ios/Client/Frontend/Browser/BraveSkusWebHelper.swift

   Lines 106 to 116 in 78fc6f6

   	guard let unescapedCredential = credential.unescape(),
   	let env = environment(domain: domain),
   	let sampleUrl = URL(string: "https://brave.com") else { return nil }
   	guard let guardianConnectCredential =
   	HTTPCookie.cookies(withResponseHeaderFields:
   	["Set-Cookie": unescapedCredential], for: sampleUrl).first?.value else {
   	return nil
   	}
   	return (guardianConnectCredential, env)

2. We should update code to store the Guardian "subscriber" credential to have that instead (I think it is named skusCredential right now; maybe it can be called desktopSubscriberCredential or similar). The credential itself along with expiration time are critical to store as a profile preference so we can easily look at it. It's very Important that this persists between app launches (which it appears to be doing already 😄)
   

   brave-ios/Client/Frontend/UserContent/UserScripts/Scripts_Dynamic/ScriptHandlers/Paged/BraveSkusScriptHandler.swift

   Lines 120 to 121 in 78fc6f6

   	Preferences.VPN.skusCredential.value = credential
   	Preferences.VPN.skusCredentialDomain.value = domain

3. Application-wise, we should not call prepare_credential_presentation unless:
   a. We don't have a credential yet
   b. We have a credential, but it's expired (now > expiration time)

4. We can set a timer to fire on the expiration timestamp we get back in Update Readme #1 above. When that expiration time is reached, we want to clear the stored credential and expiration time and fetch a new one

[Uni-verse]

Verified using 1.46 (22.12.15.19) on the following devices(s):

iPhone 12 - iOS 16.x
iPhone XR - iOS 15.x
iPad - iPadOS 15.x

• Verified credentials are valid if user relaunches the app
• Verified VPN credentials are valid if user changes servers
• Verified VPN profile is updated when changing server

Example	Example	Example	Example	Example