Return a custom TextReader from NpgsqlDataReader.GetTextReader if the string is already in the buffer

[vonzshik]

Right now we return StreamReader, but it has a problem that it allocates a 1k buffer in the constructor, with no way to change this behavior.

npgsql/src/Npgsql/Internal/TypeHandlers/TextHandler.cs

Line 273 in 521af97

public virtual TextReader GetTextReader(Stream stream) => new StreamReader(stream);