Add alternative YAML style information definition API

[braingram]

Currently the schemas are used to provide YAML serialization specific configuration:

• propertyOrder
• flowStyle
• style

This leaves users with no control over how their data is serialized (without modifying a schema which is problematic). The above keywords are constrained by the yaml metaschema (see links above) and those may be removed from the metaschema (see asdf-format/asdf-standard#256).

In practice this allows (via the schema) assignment of a style to a tagged object. One alternative option would be to define this via the converters. Let's say we allow converters to have style information, something like Converter.style.flowStyle (name and implementation to be determined). During serialization the converter would add this flowStyle to the produced tagged object. A developer can configure this via the converter.

We could consider allowing users to configure this via:

af.extension_manager.get_converter_for_type(np.ndarray).style.flowStyle = "block"

or perhaps indirectly:

af.set_default_style(np.ndarray, "block")

to allow asdf to manage the context for this (to limit the scope of this modification).