Add SQL Managed Instance support and sorting (#7996)

- Add SQL Instances folder and support using existing SQLClient API
- Sort subscriptions in tree and quickpick for easier search
- Sort all resources (Databases, Servers, Instances) alphabetically too

Not in this PR:
- Will experiment with Graph API for faster perf & easier addition of other Azure resources such as PostgreSQL

extensions/azurecore/resources/light/sql_instance.svg

@@ -0,0 +1,8 @@
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