Add IdentityIndex property to PoseIdentity

[bruno-f-cruz]

This PR adds an extra field to the PoseIdentity class that stores the ArgMax value from the Identity inference output vector when running the PredictPoseIdentities operator:

sleap/src/Bonsai.Sleap/PredictPoseIdentities.cs

Line 172 in c4a3e19

var maxIndex = ArgMax(idArr, iid, Comparer<float>.Default, out float maxScore);

This value is useful when the user wants to store the output of the network in a non-text format such as a flat binary file or HarpMessage objects.

For full backward compatibility, an [XmlIgnore] attribute was added to the property, guaranteeing that downstream operations', namely text-based logging using CsvWriter, behavior remains unchanged.

Finally, if the operator's property IdentityMinConfidence is greater than the inferred maximum confidence, the PoseIdentity class will be returned as:

IdentityIndex = -1
Confidence = float.NaN;
Identity = string.Empty;