Add a new datapoint to validator stats that shows average validators commission

Add a new data point to the validator stats outputs that shows what is the average commission on a network. This might be useful to compare the validator's commission rate against the average commission in the network.

Start by locating the validator stats output entry points and reviewing how existing datapoints are calculated and exposed. Add an average network commission datapoint, then verify that the validator's commission can be compared with it in the stats output.

Written by the indexing model from the issue text.