Deduplicate header Separator logic

[MihaZupan]

We have logic like this duplicated in 7 places:

string? separator = null;
if (headerValues.Length > 1)
{
    HttpHeaderParser? parser = header.Key.Parser;
    if (parser != null && parser.SupportsMultipleValues)
    {
        separator = parser.Separator;
    }
    else
    {
        separator = HttpHeaderParser.DefaultSeparator;
    }
}

This PR adds helpers that hide this. I also avoided the overhead of checking SupportsMultipleValues and doing the transcoding on the hot path.

As far as I can tell neither we nor Kestrel are using the HPack/QPack methods I removed which don't accept a value encoding.

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[wfurt]

LGTM