[VS Code Extension] Provide possible SKU values in autocomplete

**Is your feature request related to a problem? Please describe.**
Resources snippets are helpful to get started when creating a new resource, but I have no ideas what kind of SKU values I can choose, for example here:


**Describe the solution you'd like**
It would be nice if the extension would be able to query the list of available SKU options (as I imagine it's something that can change over time) and present it during autocomplete, even with some delay.

This would be especially useful as I have no idea where to find that information, as the templates docs (https://docs.microsoft.com/en-us/azure/templates/microsoft.web/serverfarms?tabs=bicep#template-format) do not provide it. Currently, I use the portal to create the resource I need and convert the ARM templates generated with the Bicep decompiler, which is far from ideal and convenient.

Start with the VS Code extension's autocomplete flow and the resource snippets mentioned in the issue; no file or test entry point is provided. Determine how available Azure SKU options can be obtained and surfaced during completion, then verify that resource authoring offers current SKU values without requiring portal-generated templates.

Written by the indexing model from the issue text.