feat: add billing commands for cost summaries and resource listing

Implements 'upctl billing' commands to view billing information:

## billing summary
View cost summary by category with flexible period options:
- Named periods: 'month', 'day', 'quarter', 'year', 'last month'
- Relative periods: '3months' (3 months ago), '2weeks' (2 weeks ago)
- Relative from date: '2months from 2024-06', '+3months from 2024-01'
- Direct format: 'YYYY-MM'
- Optional --resource filter for specific resource UUID
- Optional --detailed flag for full breakdown

## billing list
List detailed billing with resource names:
- Fetches and displays resource names (servers, storage)
- Same flexible --period options as summary
- --match flag for name filtering (case-insensitive)
- --category flag to filter by resource type
- Shows both UUID and name for identification

Both commands:
- Default to current month if period not specified
- Support JSON/YAML output for scripting
- Use existing GetBillingSummary API endpoint

Addresses #339

Author: mgajda

internal/commands/base/base.go

@@ -7,6 +7,7 @@ import (
 	"github.com/UpCloudLtd/upcloud-cli/v3/internal/commands/account/token"
 	"github.com/UpCloudLtd/upcloud-cli/v3/internal/commands/all"
 	"github.com/UpCloudLtd/upcloud-cli/v3/internal/commands/auditlog"
+	"github.com/UpCloudLtd/upcloud-cli/v3/internal/commands/billing"
 	"github.com/UpCloudLtd/upcloud-cli/v3/internal/commands/database"
 	databaseindex "github.com/UpCloudLtd/upcloud-cli/v3/internal/commands/database/index"
 	databaseproperties "github.com/UpCloudLtd/upcloud-cli/v3/internal/commands/database/properties"
@@ -145,6 +146,11 @@ func BuildCommands(rootCmd *cobra.Command, conf *config.Config) {
 	commands.BuildCommand(token.ShowCommand(), tokenCommand.Cobra(), conf)
 	commands.BuildCommand(token.DeleteCommand(), tokenCommand.Cobra(), conf)
 
+	// Billing
+	billingCommand := commands.BuildCommand(billing.BaseBillingCommand(), rootCmd, conf)
+	commands.BuildCommand(billing.SummaryCommand(), billingCommand.Cobra(), conf)
+	commands.BuildCommand(billing.ListCommand(), billingCommand.Cobra(), conf)
+
 	// Zone
 	zoneCommand := commands.BuildCommand(zone.BaseZoneCommand(), rootCmd, conf)
 	commands.BuildCommand(zone.ListCommand(), zoneCommand.Cobra(), conf)

internal/commands/billing/billing.go

@@ -0,0 +1,19 @@
+package billing
+
+import (
+	"github.com/UpCloudLtd/upcloud-cli/v3/internal/commands"
+)
+
+// BaseBillingCommand creates the base 'billing' command
+func BaseBillingCommand() commands.Command {
+	return &billingCommand{commands.New("billing", "Manage billing and view cost summaries")}
+}
+
+type billingCommand struct {
+	*commands.BaseCommand
+}
+
+// InitCommand implements Command.InitCommand
+func (b *billingCommand) InitCommand() {
+	b.Cobra().Aliases = []string{"bill"}
+}