Updated cfn-postgres provider with 1 new custom:: CFN policy

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also updated readme, added missing docs and cleaned up test code so testing can happen without hardcoded credentials.

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Please note!

This is a BREAKING CHANGE!!!!
Please mark / tag is appropriately!!

this due to the reversal of the DeletionPolicy default.
Our decision / thought process was that it makes no sense to do 2 commits /deploys to drop anything in cloudformation.
If you want to keep it, set "DeletionPolicy: Retain" Otherwise it gets removed when you remove the object in cloudformation.

This prevents dangling / ghost resources that you might have to clean up manually later, and ensures that you mark everything you want to keep / safeguard appropriatly.