Extensions lack full support of type system features

[braingram]

Extensions are unable to replace the type system due to lack of support for:

• custom validators: add custom validator support to extensions #1287
• equivalent reserve_blocks hook: setting compression on individual columns of astropy table astropy/asdf-astropy#156
• block access: provisionally add block manager to serialization context #1290
• controlling array storage during serialization: controlling array storage in to_yaml_tree methods #1013

[braingram]

Another example is run_modifying_hook:

asdf/asdf/asdf.py

Line 172 in 8d1dcf3

self.run_modifying_hook("copy_to_new_asdf", validate=False)

This supports creating a new AsdfFile object from an existing (in memory) AsdfFile object by calling:

af = AsdfFile(an_existing_asdf_file_object) 

The coverage survey of downstream package tests showed that this was never used.
I found no equivalent for this hook in the new extension api.