Implement incus image alias nested block

[SpiffyEight77]

Description

This pull request implements the incus_image_alias resource.

close #221

• source code
• document
• test case

Example

Terraform Configuration

resource "incus_image" "img" {
  source_image = {
    remote = "images"
    name   = "alpine/edge"
  }

  alias {
    name        = "alpine"
    description = "Alpine Linux"
  }

  alias {
    name        = "alpine-edge"
    description = "Alpine Linux Edge"
  }
}

[stgraber]

@SpiffyEight77 how's this one going?

[SpiffyEight77]

Hi @stgraber !
I'll need two more days for testing. I modified the sync state logic for image resources.

[stgraber]

@maveonair want to review this one?

[maveonair]

@SpiffyEight77 very well done! Please have a look at my comments.

[maveonair]

With the new syntax, we probably should introduce a breaking change and remove aliases as we now have the alias blocks. @stgraber what do you think?

[stgraber]

Yep, agreed, we don't want the redundancy

[maveonair]

We have the same code block in both createImageFromSourceImage and createImageFromSourceInstance. Could you extract this into a function that can be used in all other places?

[SpiffyEight77]

Thanks @maveonair and @stgraber for confirming! I'll refactor to remove the aliases later this week.