defaultValue support for block field config

[bratvanov]

Block configs currently don't support a defaultValue option.

{
  name: 'content',
  type: 'blocks',
  label: 'Content',
  // Always adds a new block on every new doc - not what we want
  // We want the default values to be added only after manually inserting the block
  // defaultValue: [
  //   {
  //     "blockType": "Quote",
  //     "quoteHeader": "Lorem ipsum",
  //     "quoteText": "dolor sit amet",
  //   },
  // ],
  blocks: [
    {
      slug: 'Quote',
      // What we want - but defaultValue is not supported here
      // defaultValue: [
      //   {
      //     "quoteHeader": "Lorem ipsum",
      //     "quoteText": "dolor sit amet",
      //   },
      // ],
      fields: [
        {
          name: 'quoteHeader',
          type: 'text',
        },
        {
          name: 'quoteText',
          type: 'text',
        },
      ],
    },
  ],
},

It would be great if block configs behave more like the array field and support defaultValue on the same level as the fields option.

The benefits of supporting defaultValue in block config might not be immediately obvious in this reduced use-case scenario with only two simple text fields, as defaultValue can be added on each of the two text fields. However, this would become very useful if blocks.fields contained some more complex nested fields (especially nested array fields). In that scenario, defaultValue on the block config level would provide a way to automatically start with a complex skeleton of multiple inserted fields whenever a new block is added by providing one large defaultValue nested array.

Currently, the workaround is to wrap all fields inside blocks.fields in a group field, and then provide a defaultValue for the group field. However, this adds another parent/level of nesting, which might be undesired.