[DataFrame] Add a LoadCsv overload or other method that takes CSV content directly

[jonsequitur]

Getting from a CSV string to a DataFrame requires some ceremony involving creating a stream first. While this might not be the ideal approach from a performance perspective for larger volumes of data, it's very useful for prototyping with sample subsets of data. An API like this will also play nicely with the .NET Interactive #!value magic command.

DataFrame.LoadCsvFromString(@"col1,col2,col3
1,2,3
4,5,6
");

[pgovind]

I think this is a good idea. I'll put a PR up for this soon and we can include this in the next preview