Use StringBuilder.GetChunks in MySqlParameter.AppendBinary

[bgrainger]

If MySqlParameter.Value is set to a StringBuilder instance (#975) and MySqlCommand.Prepare() is called, the code currently calls StringBuilder.ToString() and writes it as a length-encoded UTF-8 string.

In .NET 5.0, it should be more performant to use GetChunks:

1. loop over the chunks and sum the results of GetByteCount (for a UTF8Encoder) calculate the total length of the UTF-8 string
2. write the length prefix
3. loop over the chunks and write each out (encoding each one into UTF-8)

[novak-oleksandr]

@bgrainger I can grab this one

[bgrainger]

Fixed in 1.3.9.