[MM-63556] mmctl: Add compliance export download cmd (#30576)

* add mmctl compliance export download command and tests

- Introduced `ComplianceExportDownloadCmd` to facilitate downloading compliance export files.
- Implemented the `DownloadComplianceExport` method in the Client interface for handling file downloads.
- Added unit tests for the download command, covering successful downloads, error handling for non-existent jobs, and retries on failure.
- Included end-to-end tests to validate the command's functionality.
- Updated documentation to include usage examples and options for the new command.

* don't know why this was left out

* PR comments

* adjust test for new retry logic

* refactored download fn for compliance_export and export

* fix test due to fixed logic

* docs

server/cmd/mmctl/commands/export_e2e_test.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"strings"
 	"time"
 
 	"github.com/mattermost/mattermost/server/v8"
@@ -193,7 +194,7 @@ func (s *MmctlE2ETestSuite) TestExportDownloadCmdF() {
 		cmd.Flags().Int("num-retries", 5, "")
 
 		err := exportDownloadCmdF(s.th.Client, cmd, []string{exportName})
-		s.Require().EqualError(err, "failed to download export after 5 retries")
+		s.Require().EqualError(err, "failed to download export after 5 retries: You do not have the appropriate permissions.")
 		s.Require().Empty(printer.GetLines())
 		s.Require().Empty(printer.GetErrorLines())
 	})
@@ -227,7 +228,7 @@ func (s *MmctlE2ETestSuite) TestExportDownloadCmdF() {
 		defer os.Remove(downloadPath)
 
 		err = exportDownloadCmdF(c, cmd, []string{exportName, downloadPath})
-		s.Require().EqualError(err, "failed to download export after 5 retries")
+		s.Require().EqualError(err, "failed to download export after 5 retries: Unable to find export file.")
 		s.Require().Empty(printer.GetLines())
 		s.Require().Empty(printer.GetErrorLines())
 	})
@@ -252,7 +253,8 @@ func (s *MmctlE2ETestSuite) TestExportDownloadCmdF() {
 
 		err = exportDownloadCmdF(c, cmd, []string{exportName, downloadPath})
 		s.Require().Nil(err)
-		s.Require().Empty(printer.GetLines())
+		s.Require().Len(printer.GetLines(), 1)
+		s.Require().True(strings.HasPrefix(printer.GetLines()[0].(string), "Export file downloaded to "))
 		s.Require().Empty(printer.GetErrorLines())
 	})
 
@@ -273,7 +275,8 @@ func (s *MmctlE2ETestSuite) TestExportDownloadCmdF() {
 
 		err = exportDownloadCmdF(c, cmd, []string{exportName, downloadPath})
 		s.Require().Nil(err)
-		s.Require().Empty(printer.GetLines())
+		s.Require().Len(printer.GetLines(), 1)
+		s.Require().True(strings.HasPrefix(printer.GetLines()[0].(string), "Export file downloaded to "))
 		s.Require().Empty(printer.GetErrorLines())
 
 		expected, err := os.ReadFile(exportFilePath)