utils.serialize: modify_yaml contextmanager to reduce boilerplate

[skshetry]

Reduce boilerplate of create + load + dump during serialization/deserialization

with modify_yaml(config_path, tree=repo.tree) as config:
    config["repos"] = config.get("repos", [])
    config["repos"].append(entry)

• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

On top of #4425, see #4415 (comment)

[efiop]

💅