Allow File Rules to leverage file format metadata

Currently the OCIO File Rules are only based on the path string for the media. This was done intentionally since dealing with file format metadata was deemed too complex for the first pass at the feature. However, it's definitely the case that there often is metadata in the file header that could be useful for determining what the appropriate color space is and it would be useful for the File Rules to be able to leverage it.

We do not want to add any dependencies for file format libraries on to OCIO to support this feature, so it needs to be implemented in such a way that the host application parses the files and provides the metadata in some neutral representation, such as a pair of field & value strings. OpenImageIO has some techniques for representing file-format agnostic metadata that might be a good model for how this could work (without adding a dependency on OIIO).

Of course the metadata in file headers is often wrong, so there needs to be sufficient control in the File Rules to control the relative precedence of what is in the file relative to the existing path-based rules.

Here is a presentation that [discussed OpenEXR](https://docs.google.com/presentation/d/1sY55UHYi2AJF6EPYDYY-mvaJUCzZCkWisrj-a9yhg2w/edit?usp=sharing) that is related.

Bắt đầu bằng việc xem xét các OCIO File Rules hiện có và bài thuyết trình OpenEXR được liên kết. Xác định cách biểu diễn metadata về trường và giá trị do host cung cấp, cũng như cách kiểm soát thứ tự ưu tiên của chúng so với các quy tắc dựa trên đường dẫn. Công việc được coi là hoàn tất khi File Rules có thể sử dụng metadata trung lập mà không thêm các dependency vào thư viện định dạng tệp.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.