Add network ACL resource

[maveonair]

This pull requests adds the network ACL resource (see #78).

Examples

resource "incus_network_acl" "this" {
  name        = "my-acl"
  description = "foo"
}

resource "incus_network_acl" "this" {
  name        = "my-acl"
  description = "foo"

  egress = [
    {
      action           = "allow"
      destination      = "1.1.1.1,1.0.0.1"
      destination_port = "53"
      protocol         = "udp"
      description      = "DNS to cloudflare public resolvers (UDP)"
      state            = "enabled"
    },
    {
      action           = "allow"
      destination      = "1.1.1.1,1.0.0.1"
      destination_port = "53"
      protocol         = "tcp"
      description      = "DNS to cloudflare public resolvers (TCP)"
      state            = "enabled"
    }
  ]

  ingress = [
    {
      action           = "allow"
      source           = "@external"
      destination_port = "22"
      protocol         = "tcp"
      description      = "Incoming SSH connections"
      state            = "logged"
    }
  ]
}

What's been done

• Added network_acl.go which implements the network ACL resource
• Added network_acl_test.go which implements various tests for the network ACL resource
• Added documentation of the network ACL resource