Add Skeleton loading for side panel

[Bonapara]

Current behavior

Today, the side panel can take several seconds to appear when a user clicks on "Create Task", "Create note" or "Create relation".

See #7067

Desired Behavior

We want to introduce a Skeleton loading for the side panel to catch situations when the side panel takes a very long time to load.

https://www.figma.com/design/xt8O9mFeLl46C5InWwoMrN/Twenty?node-id=40133-444019&node-type=frame&t=U9NEkHgDVg32oWUi-11

Change the object header background color to Secondary.

[subhamengine]

hey, i would love to work upon it.

[Bonapara]

We will handle this one internally @subhamengine, thanks! You can grab a Good First Issue