MM-38698: Add limit to the CLI compliance export command (#20997)

```release-note
A batchSize option has been added to the mattermost export
CLI command to limit the number of items exported. By default,
if it is not included, it exports all the posts.
```

https://mattermost.atlassian.net/browse/MM-38698


Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

store/storetest/compliance_store.go

@@ -175,6 +175,11 @@ func testComplianceExport(t *testing.T, ss store.Store) {
 	assert.Equal(t, cposts[0].PostId, o1.Id)
 	assert.Equal(t, cposts[3].PostId, o2a.Id)
 
+	// Test limit
+	cposts, _, nErr = ss.Compliance().ComplianceExport(cr1, model.ComplianceExportCursor{}, 2)
+	require.NoError(t, nErr)
+	assert.Len(t, cposts, 2)
+
 	cr2 := &model.Compliance{Desc: "test" + model.NewId(), StartAt: o1.CreateAt - 1, EndAt: o2a.CreateAt + 1, Emails: u2.Email}
 	cposts, _, nErr = ss.Compliance().ComplianceExport(cr2, model.ComplianceExportCursor{}, limit)
 	require.NoError(t, nErr)