Add multi-version support to get_field for bigquery

[rlh1994]

Description & motivation

Add multi-version support to getfield for bigquery, just checking for an * and coalesce across those. Need to update the readme still.

Checklist

• [x] I have verified that these changes work locally
• [x] I have updated the README.md (if applicable)
• [x] I have added tests & descriptions to my models (and macros if applicable)