Add a way to Reset and Retrieve a current sessions expiry

[Badgerati]

At the moment there isn't a way to reset a current session's expiry datetime, nor a clean way to retrieve the datetime the current session will expire.

This issue is to add, at least, the new functions:

• Reset-PodeSessionExpiry
• Get-PodeSessionExpiry

[Badgerati]

The above commit adds the two functions mentioned originally, but also adds a new Get-PodeSessionDuration - so there's a way to retrieve the duration defined for each session afterwards.

I've also cleaned up the code around sessions and session cookies, including fixing a bug around non-extending sessions where their expiries were actually extending! I've also updated the Set-Cookie header builder to use StringBuilder rather than += - plus a couple other small performance tweaks.