[SPARK-18710][ML] Add offset in GLM

## What changes were proposed in this pull request?
Add support for offset in GLM. This is useful for at least two reasons:

1. Account for exposure: e.g., when modeling the number of accidents, we may need to use miles driven as an offset to access factors on frequency.
2. Test incremental effects of new variables: we can use predictions from the existing model as offset and run a much smaller model on only new variables. This avoids re-estimating the large model with all variables (old + new) and can be very important for efficient large-scaled analysis.

## How was this patch tested?
New test.

yanboliang srowen felixcheung sethah

Author: actuaryzhang <actuaryzhang10@gmail.com>

Closes #16699 from actuaryzhang/offset.