Add extra_fields to extensions.item_assets.AssetDefinition

[duckontheweb]

The Item Assets Definition Extension states:

Other custom fields, or fields from other extensions may also be included in the Asset object.

However, there is not currently a Pythonic way to do this in PySTAC. The AssetDefinition.create and AssetDefinition.apply methods only accept title, description, media_type and roles arguments, so the only way to add something like a raster:bands field to the Asset Definition is by initializing it with a dictionary that includes that property.

It seems like it would be helpful to have an extra_fields argument to both create and apply that allows additional properties to be passed in.

[m-mohr]

Oh yes, this was very confusing at the beginning and makes the code structure a bit weird. +1 on this one for sure.