Add timeout to Backup.create()

[tmatsuo]

The Backup API surface doesn't pass timeout to the underlining API call:

python-spanner/google/cloud/spanner_v1/backup.py

Line 178 in 1d49766

def create(self):

Can you add timeout param and pass it to the API call so that I can control the deadline? We occasionally see DeadlineExceededError upon Backup.create() calls.

[tmatsuo]

Once #68 is done, this issue might be a moot.

[larkee]

Now that #68 has been fixed this shouldn't be necessary. However, it does highlight the lack of custom timeout and retry support we have through our surfaces. I'm going to close this issue and open a more general one.