Converter type indexing is incompatible with some inheritance structures

[braingram]

Class factories and other inheritance patterns can result in dynamic subclasses that can be serialized with the same tag.

• sunpy generic map
• astropy Masked objects

Currently asdf expects converters to list all types at index time (when the extension manager is created often when the first AsdfFile instance is made). For the above patterns this is problematic because:

• subclasses might be defined after the converter is indexed (and there's no way to list the subclass ahead of time)
• returning the subclasses as Converter.types may require importing a module (which impacts performance) to inspect which subclasses are defined

For these patterns we may want to consider a different way of indexing converters.