Add loading indicator for Form component

[helen-m-lin]

Is your feature request related to a problem? Please describe. As a user, I want to see a loading indicator when a schema is being loaded so I know the application is not frozen.

• Most schemas render almost instantaneously so the user experience is relatively smooth
• However, longer/ more complex schemas (e.g. rig schemas) may take a few seconds to fetch and pre-process before rendering. The loading time will also be dependent on the user's network connection.
• For schemas that take longer to load, the screen may appear frozen since nothing happens right after selecting a new schema/version.

Describe the solution you'd like Add a loading indicator on the form component to indicate that it is loading and that the screen is not frozen.

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered.

Additional context Add any other context or screenshots about the feature request here.

[helen-m-lin]

For nested annotated unions (mainly related to manufacturers in rig.json and instrument.json), there is also sometimes a noticable rendering delay. Consider also adding loading indicator to nested subschemas.