v0.14 - StreamField callables

This release adds the much missed ability to use custom StreamField blocks callables or properties that is used aplenty with Page models.

class TextWithCallableBlock(blocks.StructBlock):
    text = blocks.CharBlock()

    graphql_fields = [
        GraphQLString("text"),
        GraphQLString("label"),
        GraphQLField("my_custom_field", source="get_custom_field"),
    ]

    @property
    def label(self):
        return "my block property"
    
    def get_custom_field(self, *args, **kwargs):
        return "welcome, callable source in StreamField"

This feature was developed by @kbayliss.