link.schema.yml is outdated

Problem/Motivation

Since #2411143: Change LinkItem schema to store URIs rather than URLs/paths/routes has been committed, the link.schema.yml need to be updated to match the new schema (property uri instead of url). Tests which try to set some default values generate a PHP fatal error.

This error occurs with the the uri property, but also for the attributes property for contrib module which add some attributes in the form element with a new widget.
If we export the configuration of a field link, we saw that the attributes property (defined under the options property) is duplicated directly at the "root configuration".

Example for field.field.node.article.field_link.yml

uuid: 03b5aa35-9594-41b3-a52d-40c8c1278fcc
langcode: en
status: true
dependencies:
  config:
    - field.storage.node.field_link
    - node.type.article
  module:
    - link
id: node.article.field_link
field_name: field_link
entity_type: node
bundle: article
label: Link
description: ''
required: false
translatable: false
default_value:
  -
    attributes: {  }
    uri: 'internal:/contact'
    title: Contact
    options: {  }
default_value_callback: ''
settings:
  link_type: 17
  title: 1
field_type: link

And the exported configuration of a field link with attributes on form element (used with link_class module)

uuid: 126a3187-6fca-4ef8-886b-44004ca835d8
langcode: en
status: true
dependencies:
  config:
    - field.storage.node.field_link
    - node.type.article
  module:
    - link
id: node.article.field_link
field_name: field_link
entity_type: node
bundle: article
label: Link
description: ''
required: false
translatable: false
default_value:
  -
    attributes:
      class: button
    uri: 'internal:/contact'
    options:
      attributes:
        class: button
    title: 'Nous contacter'
default_value_callback: ''
settings:
  link_type: 17
  title: 2
field_type: link

And so contrib modules get a fatal error if they try to fill default values for an attribute. You can see the fatal error for uri and attributes properties here https://www.drupal.org/node/2823552#comment-11756280

Proposed resolution

Change the property url to uri for field.value.link in the link.schema.yml. Add the attributes property under in field.value.link

Remaining tasks

Review patch for uri/url issue.
Discuss what to do about the attributes property ? Do we need to add it the the link.schema.yml as below

field.value.link:
  type: mapping
  label: 'Default value'
  mapping:
    title:
      type: label
      label: 'Link text'
    url:
      type: string
      label: 'URL'
    attributes:
      type: sequence
        label: 'Link attributes'
        sequence:
          type: string  
    options:
      type: mapping
      label: 'Link options'
      mapping:
        query:
          type: sequence
          label: 'URL query key value pairs'
          sequence:
            type: string
        fragment:
          type: string
          label: 'URL fragment'
        absolute:
          type: boolean
          label: 'Is this URL absolute'
        https:
          type: boolean
          label: 'If the URL should use a secure protocol'
        attributes:
          type: sequence
          label: 'Link attributes'
          sequence:
            type: string

User interface changes

None

API changes

None

Data model changes

What are the consequences if we add the attributes property ?