[enhancement] New message storing data points in EstimatorModel

### What's hard to do? (limit 100 words)

Currently, `message EstimatorModel` uses `message DataPoint` to store data points for an estimator model. While an estimator model is supposed to estimate exactly one metric, `message DataPoint` retains all information from synthesis, such as area and delay for an op.

### Current best alternative workaround (limit 100 words)

N/A

### Your view of the "best case XLS enhancement" (limit 100 words)

We should add another message recording value and value_offset of one metric instead of reusing `message DataPoint` in `message EstimatorModel`.