Built-in R2 (R-squared) metric

Description

LightGBM should add a built-in R^2 metric.

Benefits of this work

• prevents the need to implement this metric in a custom function (as seen in #6982, for example)

Acceptance criteria

• LightGBM supports an r2 metric (on both CPU and GPU)

Approach

Do something like this to find where rmse shows up:

git grep -i rmse

And follow that as a guide for what to update.

At a minimum:

• https://github.com/microsoft/LightGBM/blob/master/src/metric/regression_metric.hpp
• https://github.com/microsoft/LightGBM/blob/master/src/metric/cuda/cuda_regression_metric.cpp
• https://github.com/microsoft/LightGBM/blob/27dcf820f30428a5e74e52e6fa0dbb271316f899/include/LightGBM/config.h#L1013
• https://github.com/microsoft/LightGBM/blob/27dcf820f30428a5e74e52e6fa0dbb271316f899/R-package/R/metrics.R#L9

Notes

This should be the standard R^2, not the "adjusted" R^2 which penalizes models with more features.

See https://en.wikipedia.org/wiki/Coefficient_of_determination