Extend file block functionality in the instance resource to provide more granular control

[tregubovav-dev]

Problem

terraform-provider-incus provided only subset of /1.0/instances/{name}/files functionality. It allows only to upload regular files and update owners and permissions.
Incus API provides additional functionality:

• manipulate with directories and symlink files
• support file deletion

Proposal

To extend file block configuration in incus_instance resource with attributes:

• type (optional) with values:
  • file (default), covers already implemented functionality
  • directory, manipulates with the single directory. Additional validation: contentattribute must benull`
  • symlink, manipulates with the symlink file. Additional validation: content attribute must be one-line string.
• operation (optional) with values:
  • override (default), creates new file, symlink, or directory if it not exists; overrides content of regular file or changes link target for symlink if it exists; keeps existing directory if it exists.
  • append, appends content of file if it's exists. Probably additional discussion about how to handle symlink, directory or non-exist file is needed .