Refactor slo module. Update monitor-based SLOs to be able to create corresponding monitors

[aknysh]

what

• Refactor slo module
• Update monitor-based SLOs to be able to create corresponding monitors
• Add example of monitor-based SLO
• Update README

why

• Separate metric-based SLOs and monitor-based SLOs into different files for readability
• When creating monitor-based SLOs, allow specifying and creating the corresponding monitors and assigning them to the SLOs (in YAML config for each SLO)

  # For each monitor-based SLO, either `monitor_ids` or `monitors` should be provided
  # `monitor_ids` is a list of externally created monitors to use for the monitor-based SLO
  # If `monitors` map is provided, the monitors will be created by the module and assigned to the SLO

references

• Service Level Objectives
• Monitor-based SLOs

[aknysh]

/test all

[aknysh]

/test all

[danjbh]

lgtm!