RDS: implement CopyTags and Tags parameters for copy_db_snapshot()

[snordhausen]

Testing on AWS showed that tags are never merged. Instead, AWS uses either the provided Tags value or the tags of the original snapshot.
Also, the old code always copied the tags of the source snapshots, but CopyTags actually defaults to False.

I'll apply the same change to copy_db_cluster_snapshot once this PR is merged. It behaves in the same way as copy_db_snapshot with regards to tags.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.57%. Comparing base (10bdd08) to head (fb550e7).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8320   +/-   ##
=======================================
  Coverage   94.57%   94.57%           
=======================================
  Files        1159     1159           
  Lines      100936   100936           
=======================================
+ Hits        95458    95459    +1     
+ Misses       5478     5477    -1     

Flag	Coverage Δ
servertests	28.89% <0.00%> (ø)
unittests	94.54% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[bblommers]

Nice! This is a very clean and clear way of showing the different behaviours.

Just one nitpick - can you use this list to parametrize the entire test? That is going to include a bit of overhead, but having separate tests for each use case makes debugging later on much easier (IMHO).

So the resulting test signature would look something like this:

@pytest.mark.parametrize("kwargs,expected_result", [..])
def test_copy_db_snapshots_copytags_and_tags(kwargs, expected_result):
    ...

[snordhausen]

@bblommers I'll take at this eventually, just didn't have time yet.