Add support for multi-valued headers

[baywet]

Bulk of the conversation here.
We need to rely on a structure other than a dictionary in request information and request configuration that would handle multiple values and folding when generating the request, as well as headers casing.

[andrueastman]

HttpHeaders is held in a collection of KeyValuePair<IEnumerable<string> that could possibly be helpful.

https://learn.microsoft.com/en-us/dotnet/api/system.net.http.headers.httpheaders?view=net-7.0#definition

If the specified header does not exist, the Add method inserts a new header into the list of header name/value pairs.
If the specified header is already present, values are added to the comma-separated list of values associated with the header.

[baywet]

Thanks for the suggestion, I've always found that API clunky at best.
I've drafted an API for dotnet, let me know what you think: microsoft/kiota-dotnet#55