feat: add launcher_config_group parameter

[Marius1311]

Add launcher_config_group parameter to control the Hydra config group
path for launcher configs referenced by _launcher_.

Problem: _make_launcher() hardcodes hydra/launcher=<name> as a
hydra-level override. This breaks when launcher configs live at the app
level (configs/launcher/ with @package _global_), which is a valid
and common Hydra pattern.

Fix: New launcher_config_group parameter (default: "hydra/launcher")
controls where the override is placed:

• "hydra/launcher" (default) — standard Hydra layout, override goes into hydra_overrides
• "launcher" — app-level config group, override goes into task_overrides

Usage (in sweeper config):

hydra:
  sweeper:
    launcher_config_group: launcher  # for app-level launcher configs
    combinations:
      - {model: large, _launcher_: euler_gpu_8}
      - {model: small, _launcher_: euler_cpu}

[codecov]

Codecov Report

❌ Patch coverage is 8.33333% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.00%. Comparing base (b40076d) to head (8a67ebc).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...c/hydra_plugins/hydra_sweeper_explicit/_sweeper.py	8.33%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #2      +/-   ##
==========================================
- Coverage   76.00%   70.00%   -6.00%     
==========================================
  Files           3        3              
  Lines         100      110      +10     
==========================================
+ Hits           76       77       +1     
- Misses         24       33       +9     

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