Batch requests for multiple App Configuration references in App Services

App Config references in Azure App Service are currently resolved individually, with each reference triggering a separate GET request to AppConfig service. However, since AppConfig supports retrieving multiple key-values in a single request, AppService platform could optimize performance by batching these references into fewer GET requests. This would allow users to reference more AppConfig key-values without hitting throttling limits.

Based on customer feedback:

> I'm hitting our App Configuration Service (ACS) from an Azure Function using app configuration references. I notice that a request appears to be made to ACS _for each app setting individually_, ie the config retrieval is not batched into a single call.
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> I can retrieve all of our app config keyvalues using azure cli eg
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> ```
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> az appconfig kv list --key "/ian/api/dev/*" --connectionstring "Endpoint ..."
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> ```
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> where "/ian/api/dev/" is the common prefix of all of my app config key names.
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> Obviously the key names _could_ be completely different with no common prefixing.
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> But I wonder whether some logic could be built into the "fetch" code that checks for common prefixing and retrieves a whole batch (up to 100 entries?) in a single call. Retrieved keyvalues could then be matched to app settings based on their full keyname.
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> Would eliminate some of the 429s we're seeing (even in Standard tier).
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> Thanks!

_Originally posted by @ianrandell-sh in [#62](https://github.com/Azure/AppConfiguration/issues/62#issuecomment-1557464451)_