Refactor DistributedBy

Context:

With OPT-3315, we want to add distributed by variable and distributed by start/end date reports. To create these distributedBys, we need additional information about the variable (name/type) and date (bucket unit). Currently, the distributedBy field in the report config is just a type which does not include any of this context info because so far, we did no need it for the report types we have. 

In order to create the above mentioned reports, we need to adjust our distributed By to be similar to our `Decision/ProcessGroupByDto<?>`, that means:

• The current `DistributedBy` field in the config becomes `DistributedByType`
• A `ProcessDistributedByValueDto` is added
• A `DistributedByDto<Value extends ProcessDistributedByValueDto>`is added
• For the mentioned reports we will also need (this can be implemented with the sub tasks of OPT-3315)
  • `VariableDistributedByDto`
  • VariableDistributedBValueDto`
  • `StartDateDistributedByDto`
  • `EndDateDistributedByDto`
  • `DateDistrbutedByValueDto`

This refactoring will be split into two parts: part 1 to adjust the current distributedBy field to allow for distributed by Variable and date reports and part 2 to move this distributedBy field from the ReportConfiguration to the ReportData.

AT:

• The current distributedBy field is refactored to be a `DistributedByDto<Value extends ProcessDistributedByValueDto>`
• This field is moved from the config to the ReportData
• All existing reports are migrated 
• The documentation is updated
• The frontend is adjusted accordingly