Add zstd as a compression option for the OpenEXR plugin in OIIO

[TodicaIonut]

Description

After adding the new zstd compression codec to OpenEXR, here we enable it in the OIIO stack so it will be recognized by the library and binary tools, such as using oiiotool to directly change compression type into zstd.

I'm also trying to study if more changes on the OIIO side is required to enabled the new zstd codec throughout the OIIO tool chain... suggestions welcome.

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[lgritz]

I think you have an error.

(only the "bleeding edge" test is building against OpenEXR 'main', so that's the only one that's really enabling your changes)