Add router context manager utility for asserting call count

e.g.

```py
@respx.mock
def test_called():
    respx.get("https://example.com/foobar", name="foobar")
    respx.get("https://example.com/hamspam", name="hamspam")

    with respx.assert_call_count(foobar=1, hamspam=0):
        httpx.get("https://example.com/foobar")
```