Allow for overrides when coverting

[ian-bartholomew]

Problem to solve

Nobl9 requires a few things that the OpenSLO spec does not support. However, for the use case of someone keeping the source of truth in OpenSLO and converting to Nobl9, we don't have a good way of providing those values, outside of manual updating.

Example:
The MetricSourceSpec here has Name and Kind that we need to add when assigning here

Proposal

It would be ideal to have a way to provide inline attributes that are not part of the OpenSLO spec to give hints to the converter to include in the output. We don't want to add these to the spec structs tho, since they are not part of the spec.

Another option is to use something like yk to merge the output with an override file, though this way is less than ideal since its brittle