Issue 51 pcu group methods (#52)

* Add pcuGetTypes and pcuGroupGetByDatacenterUUID endpoints

- POST /v2/pcus/types (pcuGetTypes): get available PCU types with optional region/provider filters
- GET /v2/pcus/actions/get/{datacenterUUID} (pcuGroupGetByDatacenterUUID): get PCU group for a given datacenter
- New schemas: PCUGroupTypesRequest, PCUGroupTypeResponse, PCUGroupTypeDetails, PCUGroupType
- Use '500' literal response codes (not 5XX wildcard) consistent with repo convention
- Regenerated astra.gen.go

* fix: restore 5XX on pre-existing paths; scope 500 to new PCU endpoints only

Author: bdunn313