Add support for multiple ion servers in UI

[lilleyse]

Fixes #562.

Follow up to #578 that adds a cesium ion server selector in the UI. There are still a few UI components that are not addressed in this PR but will come in a follow up:

• Update token selector to support multiple ion servers #563
• Update Asset Window to support multiple ion servers #565
• Dynamically populating the Quick Add assets based on the server

In addition, there are some bugs related to saving and loading:

• Saving a .usda with multiple servers for the first time causes a server disconnect
• Loading a .usda with multiple servers doesn't resume the connection

Projects with the single default ion server don't seem to have these issues, which is good.

Also...

• Creating a new project always requires connecting to ion. That's because there's no existing connection to resume. I think this is expected behavior, but would have to verify with Cesium for Unreal.

Finally, while I started to implement something that would query the ion api url automatically, this didn't really work with the existing code. @r-veenstra any tutorial content will need to be updated so that the api url is filled in instead of left blank. E.g. for https://self-hosted.cesium.com/ it would be https://api.self-hosted.cesium.com/.

Obviously there are a lot of improvements to make here still, but it's certainly usable. Here I add CWT from ion.cesium.com and GTOPO from self-hosted.cesium.com.

self-hosted-omniverse.mp4

[r-veenstra]

Okay, this all appears to be working in my testing, with exception to the bugs and limitations outlined in the PR.

I have noticed one additional issue where the "Create a new token" option in the Token UI doesn't appear to create tokens with self hosted, but works ok with regular ion.

[lilleyse]

Creating a new project always requires connecting to ion. That's because there's no existing connection to resume. I think this is expected behavior, but would have to verify with Cesium for Unreal.

This is fixed in #594. It should no longer sign you out each time. What was happening was that the property change notifications here were triggering a disconnect here. Since property change notifications are now ignored after prim add notifications, this no longer happens.