Allow specifying the sla_type viewed

For gitlab-com/gl-infra/scalability#1659 (comment 918788859)

In the previous MR, !4533 (merged), we added some more versions for SLA calculation, including a template to the SLA general dashboard. However, we reverted the change due to a duplication in the recording rules. We added the fix as well as the recording rules here: !4537 (merged). The later MR does not include the template. This MR is to add the SLA type template to aforementioned dashboard.

Before: https://dashboards.gitlab.net/d/general-slas/general-slas?orgId=1 After: https://dashboards.gitlab.net/dashboard/snapshot/YTG7lgwYjXdXuEVPb3BW5gdOblKdz3nc?orgId=1&from=now-2d&to=now