Honoring LONGBLOB column type

[psteinb]

Hi,

thanks for providing this handy julia module to interface with MySQL DBs.

In my application, I have the need to store larger byte strings, aka Vector{UInt8} type columns. If I understand the library design correctly, all Vector{UInt8} columns of a DataFrame object are mapped to a MySQL BLOB column type - see src/api/consts.jl and src/load.js. However, the MySQL standard supports quite a handful of BLOB types (see here)

1. Would it be possible for me, to mixin support for LONGBLOB in any way without having to change the added module in my project?
2. Would you be open to add LONGBLOB support?
   From scanning the code, I personally don't see any obstacles to that. But I consider myself a beginner and hence am perhaps overlooking something. I am happy to send a PR, if the change is of manageable difficulty.

Best,
P