Add scale sets

[gabriel-samfira]

This PR adds scale set support to GARM.

Adding a new scale set is identical to adding a pool:

garm-cli scaleset add \
    --repo  30c6c4c7-e020-4203-b4f5-80f857b93238 \
    --provider-name yourProviderName \
    --image ubuntu:22.04 \
    --name test-scale-set \
    --flavor default \
    --enabled=true

Scalesets have the same commands that pools have:

ubuntu@garm:~$ garm-cli scaleset --help
Query information or perform operations on scale sets.

Usage:
  garm-cli scaleset [command]

Available Commands:
  add         Add scale set
  delete      Delete scale set by ID
  list        List scale sets
  show        Show details for a scale set
  update      Update one scale set

Flags:
  -h, --help   help for scaleset

Global Flags:
      --debug           Enable debug on all API calls
      --format string   Output format (table, json) (default "table")

Use "garm-cli scaleset [command] --help" for more information about a command.

Runners will appear along side the pool runners in:

garm-cli runner ls -a

[gabriel-samfira]

This still needs a lot of tests, but marking it as ready for review, as it should be fairly functional if anyone feels brave enough to test it.

[gabriel-samfira]

This has grown quite a bit. Merging this as is and will propose new PRs for additional tests and docs updates.