Add API for shareable drafts

[jfreda]

This PR adds an API for sharing drafts documents with the Google Workspace organization.

Breaking changes

• New required google_workspace.domain config attribute:

google_workspace {
  // domain is the Google Workspace domain (e.g., "hashicorp.com").
  domain = "your-domain-dot-com"
}

Note: For document drafts that are stored in shared drive folders in Google Workspace, this feature may need "Allow people who aren't shared drive members to access files" checked in the Shared drive settings for the shared drive. The screenshot attached shows the Shared drive settings.

New API endpoints

• /api/v1/drafts/{document_id}/shareable
  • Methods: GET, PUT

[anubhavmishra]

Minor comments and suggestion! Overall looks good!