add uncapped_max_size feature to allow increasing max BSON size

[Trevader24135]

For various projects I'm working on, I'm using BSON as an efficient means to transfer potentially quite a bit of data (~200Mb). From what I can read of the BSON standard, it's expected to support document sizes up to 2Gb, but this implementation is currently hard capped to 16Mb to better service MongoDB.

I figured adding an opt-in feature to increase the cap to the maximum would serve to be useful for more people than just me, without affecting the way that it's currently used within MongoDB applications.

I ran all the unit tests and linting tests and didn't encountered any failures with or without the added feature enabled.

Let me know if I'm off-base thinking that this is a useful addition, or if there are any unforseen issues with expanding the max size. I haven't done extensive reading of this library, so I may be missing something else that needs accounted for.

Thanks!