Add a spinning icon when previewing a table

[dfalbel]

Addresses: https://github.com/posit-dev/positron/issues/10835

Release Notes

New Features

• A spinning icon is shown when the connections pane is loading a table preview (https://github.com/posit-dev/positron/issues/10835).

Bug Fixes

• N/A

QA Notes

To reproduce, create a connection with eg BigQuery which can be quite slow at previewing and then click on to preview a table:

https://github.com/user-attachments/assets/db5f63e2-8b07-448e-8cc8-d6ca173a7b78

[github-actions[bot]]

E2E Tests 🚀 This PR will run tests tagged with: @:critical

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