[v15] Include an optional screen size in the WindowsDesktop spec

[zmb3]

By default this should remain unspecified, which preserves the behavior we have today (Teleport uses the size of the user's browser window).

These new fields allow cluster administrators to configure desktops in such a way that they always use a particular screen size, ignoring the size passed from the browser client. This is necessary to accommodate some applications which require a very specific screen resolution in order to render correctly.

changelog: Desktops can now be configured to use the same screen resolution for all sessions.

Backports #34580

[zmb3]

Holding this open until we're closer to the v15.1 release at the end of the month.