Support for name/value pairs

As part of the discussions around issue #1975, there has been a side discussion around allowing arbitrary name/value pairs. For example, many file formats allow some sort of capability like this to store metadata. OpenEXR is one example.

However, an OCIO config is not really an asset format, like an image file, so different considerations apply.

In addition, features like this can easily become unwieldy. It's difficult to keep people aligned on maintaining consistent usage. We've already seen this in OCIO with the roles mechanism where the usage has been so inconsistent that it's difficult for applications to use it for many of the things it was intended for without declaring their own private set. (Roles have been a bit of a train wreck, IMHO.)

And there may be security risks or performance issues if people are allowed to use this to include binary blobs (e.g., encoded using base64 or something similar that would work in YAML).

As with both OCIO roles and OpenEXR attributes, it would be possible to use some sort of namespace convention to organize names into groups. The namespace would indicate what organization owns the semantics for that name/value pair.

This issue is to capture specific use-cases for how people would expect this feature to be used and what applications would be expected to do with it.