[FEATURE REQUEST] Global attrib to enable/disable formats

From a [slack conversation](https://academysoftwarefdn.slack.com/archives/C05782U3806/p1747335172629159?thread_ts=1747334142.587609&cid=C05782U3806), maybe it would be helpful to have a global OIIO:attribute() way to set at runtime a specific list of which file formats OIIO is willing to read or write?

This is settable at build time now, of course. But I could imagine it being helpful, especially in cases where you have to live with whichever libOpenImageIO you find on your system, to be able to restrict which formats you support. One reason might be security or reliability -- essentially limiting the attack surface area to just the formats a particular app anticipates needing?

It would be fairly straightforward to implement.