Provide polygon-to-circle utilities in python

[95-martin-orion]

In Polygon::to_oas, there is a section for converting "circular" polygons to their idealized (center, radius) representation. This behavior can be useful outside of OASIS conversion - for example, when working with imported GDS designs, or for exporting to other file formats (notably Gerber, which prompted this request: gdsfactory/gdsfactory#1466).

The request is to add the following methods to the Python API:

# In gdstk.Polygon
def as_circle(self, tolerance: float) -> tuple[tuple[float, float], float]:
    """If this polygon is a circle within `tolerance`, returns its (center, radius) representation."""