[Feature Request] Add experiment branch merge command for combining metrics and params

[fuleinist]

Problem Description

Currently, DVC provides excellent experiment tracking with `dvc exp run`, `dvc exp show`, and `dvc exp apply`. However, there is no built-in way to merge two experiment branches together, similar to how Git allows merging branches.

When working on ML projects, teams often want to:

• Combine hyperparameters from multiple experiments into one
• Merge metrics from baseline and variant experiments
• Create "ensemble" experiments from individual runs

Currently, users must manually copy values or write custom scripts to combine experiment results.

Proposed Solution

Add a new command: `dvc exp merge`

Example Usage:

```bash

Merge experiment exp-1 into exp-2, keeping exp-2 params where they differ

$ dvc exp merge exp-1 into exp-2

Create a new experiment combining metrics from both

$ dvc exp merge exp-1 exp-2 --name exp-ensemble --strategy combine

Merge with explicit parameter override

$ dvc exp merge exp-baseline into exp-new --force-params learning_rate
```

Use Cases

1. Hyperparameter Search: Combine best learning rates from multiple search experiments
2. Ensemble Creation: Merge metrics from different model variants
3. A/B Testing: Combine control and treatment experiment results
4. Baseline Preservation: Merge new insights into baseline without losing original

Implementation Suggestions

• Use a strategy pattern: `keep-target`, `keep-source`, `combine`, `average`
• Support `.dvc` file merging for params and metrics
• Integrate with existing `dvc exp apply` workflow
• Consider CLI flags: `--name`, `--strategy`, `--force-params`