django: proper operators for i{end,start}swith and contains
django: proper operators for i{end,start}swith and contains

Case insensitivity for Cloud Spanner is done using
Re2 syntax's case-ignore flag
    (?i)
paired with the REGEXP_CONTAINS operator, so for:

* istartswith:  REGEXP_CONTAINS(%s, r'^(?i)%s')
* iendswith:    REGEXP_CONTAINS(%s, r'(?i)%s$')

and for 'contains', use `LIKE %s` since Cloud Spanner doesn't
use 'IN' for string matches.

googleapis/python-spanner-django,googleapis/python-spanner-django