### Description
Migrates the datasets tags aggregation API `GET
/api/v1/datasets/tags/aggregation` from Python to Go.
### Changes
- Registered the `GET /api/v1/datasets/tags/aggregation` route.
- Implemented `AggregateTags` in datasets `handler` and `service`.
- Added handler and service `unit tests`.
### Test Verification
- Verified by comparing results between Python (9380) and Go (9384)
services.
- Tested scenarios: single dataset, multiple datasets, empty parameters,
and unauthorized/invalid IDs.
- All tests and Go `unit tests` passed.
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