ClientModel: Investigate whether we can remove PipelineRequestHeaders.TryGetValues( ... IEnumerable ) overload about azure-sdk-for-net

[annelo-msft]

Some notes on this investigation:

• Azure.Core RequestHeaders has a TryGetValues(IEnumerable) overload. We could move the implementation of this into Azure.Core only, but it would mean splitting the implementation of RequestHeaders across libraries.
• System.Net.Http HttpHeaders has only the TryGetValues(IEnumerable) overload, but it also has an Add(string, IEnumerable), so the type doesn't necessarily have to allocate the collection itself
• Having TryGetValue(string name, out string? value) side by side with TryGetValues(string name, out IEnumerable<string>? values) is a nice clue to the caller that the header value can be multi-valued. This doesn't appear to be common knowledge from some of the conversations I've had recently.
• We have this code in Azure.Core today, so this is an opportunity to improve performance in ClientModel, but shouldn't represent a regression to performance with the current implementation
• I wonder, if there was an efficient way to split over char buffers, if we could support the IEnumerable API without the need to allocate a collection?
• There are some nice optimizations to the System.Net.Http implementation here. Maybe we continue to support this API in the current state for now to make it easy to have a single implementation across Azure.Core and ClientModel, and come back and optimize it later?

[annelo-msft]

Closing per offline conversation