Replace the "extensions" section of the file history with a section for implementation-specific metadata

[perrygreenfield]

The "extensions" section of the history object contains a list of AsdfExtension class names used by the Python library when writing the file. This is useful when debugging issues with a file, and enables the Python library to issue warnings when an extension that was used to write the file is missing on read. Since the concept of an "extension" is not defined by the ASDF Standard and is an implementation detail of the Python library, it may not be reasonable to require that other implementations store their metadata in the same structure.

An alternative is to replace "extensions" with a new section for freeform implementation-specific metadata.

Perhaps use a standard convention for library specific metadata; e.g., some sort of standard prefix?