refactor(skore): Change `ReportType` from `Literal` to `enum.StrEnum`

[auguste-probabl]

What would you like to say?

The report type is used in many places, but because we use literal strings there is no auto-completion and the type-checking might not work properly. Using StrEnum is the right tool for this.

The report type is also used in the ComparisonReport to identify which type of report is being compared: the refactor should also touch that part of the code.

[auguste-probabl]

@jeromedockes This would be a good place to start!