fix(databases): preserve database firewall descriptions on append and remove (#1869)

[Kanika0306]

Summary

Fixes #1869: Database firewall rule descriptions were erased when appending or removing rules via doctl databases firewalls append or doctl databases firewalls remove.

Cause

When reconstructing existing firewall rules prior to invoking UpdateFirewallRules, doctl copied UUID, ClusterUUID, Type, and Value, but omitted Description. Additionally, the vendored godo.DatabaseFirewallRule struct was missing the Description field (json:"description,omitempty").

Solution

1. Added Description field to godo.DatabaseFirewallRule in the vendored godo package.
2. Preserved Description in RunDatabaseFirewallRulesAppend: Copied firewallRule.Description = rule.Description during rule reconstruction.
3. Preserved Description in RunDatabaseFirewallRulesRemove: Copied Description: rule.Description during rule reconstruction.
4. Updated DatabaseFirewallRules displayer: Added Description to Cols(), ColMap(), and KV() so descriptions render in CLI output.
5. Added Regression Tests: Added TestDatabaseFirewallRulesAppend and TestDatabaseFirewallRulesRemove to verify that existing firewall rule descriptions are preserved in the payload sent to DigitalOcean API.

Verification

Ran unit tests:

go test -v ./commands -run TestDatabaseFirewallRules

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