Feature/improvement: Ability to specify slo-plugins as a yaml file rather than json command arg

[cwld]

Currently the only way to set the slo plugins chain for all files is using the --slo-plugins or SLOTH_SLO_PLUGINS env var, which is a json spec. This is fairly clunky, especially when specifying multiple plugins.
Currently I've implemented in our pipeline some horrible bash scripting to convert a simple chain yaml with a similar format to what can be defined at the service/slo level, but it would be much easier if we could just pass in something like?

  chain:
    - id: "sloth.dev/core/debug/v1"
      priority: 9999999
      config: {msg: "Plugin 99"}
    - id: "sloth.dev/core/debug/v1"
      priority: -999999
      config: {msg: "Plugin 0"}

as a file command line argument, eg
sloth generate --slo-plugins-file=<plugin-chain.yaml> ...

Would be a simple and non-breaking change as we can just add this new arg. If you're happy with this, I'd be willing to implement it.