Support custom SLO types

[freeseacher]

In google book there are more types of SLO and also custom types if needed.
Lets add support for Batch and Storage SLO

[wpjunior]

@freeseacher, I was thinking for support saturation SLOs, it means that we can define SLO for % of usage of storage, % of usage of memory, etc.

For batch SLO, do you have an example ?