Custom quadrature rules, runtime quadrature

There are few code paths for runtime quadrature. Unfortunately, this is identified with a custom UFL measure, while I think it should be just stored in measure's metadata.

One example interface for custom and runtime quadrature could be
```python
v * dx(metadata={"quadrature_degree": 2, "quadrature_scheme": "whatever_preimplemented"})
v * dx(metadata={"quadrature_scheme": "custom", "quadrature_points": [[1.0, 0.5], ...], "quadrature_weights": [1.0, ...]})
v * dx(metadata={"quadrature_scheme": "runtime"})
```

FFCx signature for runtime quadrature (custom integral type) needs updating.