ENH: Allow updating GBQ table schemas
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14694?src=pr) is 85.24% (diff: 12.00%)
> Merging [#14694](https://codecov.io/gh/pandas-dev/pandas/pull/14694?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **0.04%**

```diff
@@             master     #14694   diff @@
==========================================
  Files           143        144     +1   
  Lines         50787      50941   +154   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43273      43426   +153   
- Misses         7514       7515     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [f26b049...7056a7a](https://codecov.io/gh/pandas-dev/pandas/compare/f26b049786624ed983f2718687c23e3f1adbb670...7056a7ac9546dd81ce983ae845e231e5a0b7f550?src=pr) Looks like the 1 failing test was because of a conda repo issue.
 what is the purpose of this? is there an issue for this change? @jreback There isn't a tracked issue.  I was running into an issue where I wanted to append to existing GBQ tables but either had a new column or were missing a few legacy columns and that caused the append to fail. cc @parthea thoughts? status of this? I haven't had a chance to make the changes.  I'll try to get them done this week. @joshowen this has migrated to https://github.com/pydata/pandas-gbq, feel free to open issue / PR there.