Process filter in ListBudgetEntries

https://redmine.teklia.com/issues/8453

The ListBudgetEntries endpoint should accept a process_id filter, documented in OpenAPI as an optional UUID. When this is set, only the budget entries that match the process ID are shown. process_id=None is not supported.

If this filter is set to anything other than a UUID, an HTTP 400 error should be returned with {"process_id": [/* exception message */]}.

No specific access rights are required on the process. It is not necessary to check whether or not the process exists either. If it doesn't exist, the list won't return anything since the WHERE will not include anything.