Introduce `default_args` context manager (#39964)

This adds a rather trivial context manager that lets you deduplicate repeated
arguments in directives, e.g.

```python
depends_on("py-x@1", when="@1", type=("build", "run"))
depends_on("py-x@2", when="@2", type=("build", "run"))
depends_on("py-x@3", when="@3", type=("build", "run"))
depends_on("py-x@4", when="@4", type=("build", "run"))
```

can be condensed to

```python
with default_args(type=("build", "run")):
    depends_on("py-x@1", when="@1")
    depends_on("py-x@2", when="@2")
    depends_on("py-x@3", when="@3")
    depends_on("py-x@4", when="@4")
```

The advantage is it's clear for humans, the downside it's less clear for type checkers due to type erasure.

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