Add support for statistic field in CloudwatchMetricsTarget

[Butterneck]

What does this do?

Add support for statistic field in CloudwatchMetricsTarget

Why is it a good idea?

The existing statistics field is being ignored by grafana 9.x, it must be named statistic (not statistics) and must be a string (not a list)

[JamesGibo]

Thanks for this PR, I do not use cloudwatch so will have to trust that this is required, but as this is not a breaking change I am happy to add it.
Please could you add a little description of what the difference is between the 2 fields to the doc strings?

[Butterneck]

Honestly I'm not able to answer your (legitimate) questions, since I could not find any documentation about it.

I was trying to create a dashboard that uses CloudWatch for some panels and through some reverse engineering I found that setting the field "statistics" did not produce any effect (the query is considered wrong), calling it "statistic" and changing it's type from list to string, instead, works fine.

I've just added a new field instead of replacing the existing one, since "statistics" seems to have a meaning at some points (https://grafana.com/docs/grafana/latest/datasources/aws-cloudwatch/template-queries-cloudwatch/#query-variable); and I've added it just to v9 because I've tested it with this version.