provision to add custom attribute in fields

Provision to provide custom attributes to artifact type field

- Why we want to add this feature? The drive to make the change:

Currently only few predefined attributes can be configured as part of field definition. these are :
 nullable, default, mutable, required_on_activate, system, sortable

But if we need any other custom attributes to be predefined as part of field definition it is not possible as of now.

- Solution: providing custom attribtue option as part of field definition

- how: <More technical details than the high level overview of `solution` if needed.>

there are currently 2 options to provide this feature :

1. add a attribute named "custom_attribute"/"additional_attribute" of Dictionary type. which will hold all the custom_attributes related to field. As it below example "custom_attributes" is holding all excess annotations.

Ex : 'source_url': Field(fields.StringField, validators=[validators.Regex(_allowed_url_pattern)],
                            description="URL where the data for this image already resides."
                                        "Eg: http://yum.cloud-band.com/Installs/Templates/tiny/tinycentos.qcow2",
                            custom_attributes={'ui_help_key':'PACKAGE.HELP_MESSAGES.IMAGE.SOURCE_URL', 'display_name':"Source URL"}),

2. Add all attributes in linear way. without defining it inside a map. But there is a limitation that all custom field attributes has to be defined after all glare defined attribtues.

Ex : 'source_url': Field(fields.StringField, validators=[validators.Regex(_allowed_url_pattern)],
                            description="URL where the data for this image already resides."
                                        "Eg: http://yum.cloud-band.com/Installs/Templates/tiny/tinycentos.qcow2",
                            ui_help_key='PACKAGE.HELP_MESSAGES.IMAGE.SOURCE_URL', display_name="Source URL")

 DB prespective: As these are static field changes, There are no database changes required.

APIs prespective: It does not affect any API.

- impacts:
   Schema API: It will update field schema definition.

- How to use this feature from client prespective: <Cli usage example/story> :
  Based on the option we choose, we can define custom attribtues in same way as provided example above.