Handle update action for Gatsby preview

[Simply007]

Motivation

Handle edit action in Kentico Kontent from gatsby preview custom element

Format of the request when the action is performed:

{
      message: {
        operation: "update" : string,
        elementCodenames: changedElementCodenames : <string>[],
        selectedLanguage: language: string
      },
      data: { : JSON
        response  // response items/CODE_NAME
      }
}

Internal link:

• https://kentico.atlassian.net/browse/DEVREL-364

Checklist

• Code follows coding conventions held in this repo
• Automated tests have been added
• Tests are passing
• Docs have been updated (if applicable)
• Temporary settings (e.g. variables used during development and testing) have been reverted to defaults

How to test

If manual testing is required, what are the steps?

[Simply007]

TODO:

• It is required to decide what to do with usedByContentItems property because if we want to keep it and implement edit - wee need to edit all affected linked items when rich text/linked items element is changed
  • My proposal: kill it with fire and continue with v 5.0.0-beta with preview functionality and think about more of these links that source plugin is making and if they are useful - because maintenance is high for these - are you using these @jupl, @bwlng, @rshackleton
    *Think about the different type of resolution to JS SDK object
  • https://github.com/orgs/Kentico/teams/global-collaborators/discussions/9
• Remove all TODO comments from preview.js

[Simply007]

blocked by #108

[Simply007]

Implementation will be done based on the vNext branch.
Closing this as deprecated.