stores: use CreateSchema

[pjonsson]

There is already a DropSchema in
stores, so put the CreateSchema
in there as well.

The previous code has some comment
about being able to run things
without create permissions, but
the only caller is SummaryStore.init()
whose documentation states that
it requires create permissions,
so use the if_not_exists parameter
to CreateSchema instead of some
function that probes the database.

---

📚 Documentation preview 📚: https://datacube-explorer--846.org.readthedocs.build/en/846/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.52%. Comparing base (ca2727b) to head (25bac00).
⚠️ Report is 7 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #846      +/-   ##
===========================================
+ Coverage    84.48%   84.52%   +0.03%     
===========================================
  Files           35       35              
  Lines         4248     4245       -3     
  Branches       538      536       -2     
===========================================
- Hits          3589     3588       -1     
  Misses         462      462              
+ Partials       197      195       -2     

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