Use "genuine" Iceberg REST spec

[snazy]

Describe the bug

spec/rest-catalog-open-api.yaml is a modified/customized version of the "genuine" Iceberg REST spec. It can cause issues when the Iceberg REST spec introduces incompatible component names, endpoint paths, etc.

It would be cleaner to move Polaris specific changes elsewhere and leave the "genuine" Iceberg REST spec untouched.

It would also be cleaner to reference a specific & well-known version of the Iceberg REST spec instead of copying it.