Feature: Accessibility the mapping for `_operatorSets`

## Description
- The mapping for registered operatorSets for an AVS must be accessible in order to find that if `operator` is registered for a particular `avs`.
- The mapping is declared in `AllocationManagerStorage.sol`.
```bash
/// @dev Lists the operator set ids an AVS has created
mapping(address avs => EnumerableSet.UintSet) internal _operatorSets;
```
### Action Items
- [ ] Add a function returning all the `operatorSets(OperatorSet[] memory)` for an `avs` in `AllocationManager.sol`
- [ ] Change the interface `IALlocationManager.sol` accordingly

Start with the _operatorSets mapping in AllocationManagerStorage.sol, then inspect AllocationManager.sol and IAllocationManager.sol. Add the AVS operator-set listing entry point and update the interface so callers can retrieve all OperatorSet values for an AVS; done means the mapping is externally accessible through the declared interface.

Written by the indexing model from the issue text.