Add utilities to build primitive shapes

[diegoferigo]

Initial support of creating elements of primitive shapes.

Quick example:

import numpy as np
import rod
from rod.builder import primitives

link_H_com = np.eye(4)
link_H_com[0:3, 3] = np.array([0, 0, 1.0])

model = (
    primitives.SphereBuilder(name="sphere", mass=1.0, radius=0.1)
    .build_model()
    .add_link()
    .add_inertial(pose=rod.Pose.from_transform(transform=link_H_com))
    .add_visual()
    .build()
)

sdf = rod.Sdf(version="1.9", model=model)

print(sdf.serialize(pretty=True))
sdf_string = sdf.serialize(pretty=True)