[NAT-GW/SNAT] Add Description parameter

[uli-schneider]

Description of the enhancement

Console and API offers Description field for SNAT Rule.

https://docs.otc.t-systems.com/nat-gateway/api-ref/apis_for_public_nat_gateways_v2.0/snat_rules/creating_an_snat_rule.html

Please add parameter to your provider:

https://registry.terraform.io/providers/opentelekomcloud/opentelekomcloud/latest/docs/resources/nat_snat_rule_v2