Add `Mixing` pydantic model

[TimothyWillard]

To represent:

from op_system import normalize_rhs

spec = {
    "kind": "expr",
    "state": ["S", "I"],
    "equations": {"S": "-beta * S * I", "I": "beta * S * I - gamma * I"},
    "axes": [
        {"name": "space", "type": "continuous", "domain": {"lb": 0, "ub": 10}, "size": 5},
    ],
    "mixing": [
        {"name": "K", "axes": ["space", "space"], "form": "gaussian", "params": {"sigma": 1.5, "scale": 1.0}},
    ],
}

rhs = normalize_rhs(spec)

# Access normalized metadata
axes_meta = rhs.meta["axes"]        # List of normalized axis definitions
mixing_meta = rhs.meta["mixing"]    # List of normalized mixing kernels

Need a Mixing pydantic model that is a discriminated union over:

op_system/src/op_system/specs.py

Lines 53 to 60 in bb41b22

	_ALLOWED_MIXING_FORMS: dict[str, tuple[str, ...]] = {
	"erfc": ("scale", "sigma"),
	"gaussian": ("scale", "sigma"),
	"exponential": ("scale", "lambda"),
	"gamma": ("scale", "k", "theta"),
	"power_law": ("scale", "sigma", "p"),
	"custom_value": (),
	}