update metadata without rewriting the entire file

It would be nice to have the ability to update metadata on an EXR without rewriting all the channels in a file.

- This is especially arduous around very long shots (35k frame oner/one-take shots) or location-based entertainment where the shots can easily be 25-50k frames.
- Recently, with the need for machine learning where you can have 10-100k+ frames of training media. File ingestion can take days or weeks because so much media was shot, and all you want to do is copy it into place and then use it without needing to pre-process it first.

More specific use cases for this:
- Inserting metadata into a render as a post-process from the pipeline. A pipeline has to write the data twice for every frame, which is very expensive on long shots or large frames. We actually disable all metadata modifications on Deep files, for example, because OIIO needs too much memory to rewrite them every frame, and the beefy machines with lots of RAM are reserved for FX sims. This cannot be wasted on simply inserting metadata into deep exr files which can take 30-60 minutes per frame to do.
- creating a "meta" shot for the "oner" shots where you merge together 100-200 different shots into one master shot because the client wants the keys in the EXR a specific way. Delivery for these kinds of shots takes way too long to do.
- The client embedded CDLs in the header, and the CDL changed. Artists will wrongly trust the metadata and try to pull the values into their script, but this is wrong. Rewriting thousands or even millions of files from the client is expensive when things like this are found after the fact. I can also see this becoming an issue with FDL in the future when they change, too.
- republishing a plate or render from one show to another or one shot to another, and all I want to do is copy the files and then update a "SHOT" or "SEQ" key.