Support synced block creation

[BoD]

Currently synced blocks are read-only, but the API also allows creation.

Note that there are 2 possible types of creating a synced block:

• "original" synced blocks: the "source" which can contain children blocks
• "reference" synced blocks: they reference a source one, and can't contain children

It's all explained here: https://developers.notion.com/changelog/synced-block-link-to-page-and-template-block-types-are-now-supported-in-the-api