provider/aws: Add configuration to enable copying RDS tags to final snapshot

[catsby]

RDS by default does not copy any RDS Instance tags over to the final snapshot when you destroy a database. This PR adds an optional boolean to enable applying all RDS Instance tags to it's snapshot. It works in conjunction with the final_snapshot_identifier.

It doesn't look like RDS Clusters have this ability.

Unfortunately I can't seem to find how to view these tags on a snapshot in the console, but you can see them with the aws CLI, though you have to figure out the ARN:

$ aws rds list-tags-for-resource --resource-name "arn:aws:rds:us-west-2:<my account>:snapshot:<the name of my final snapshot>"
----------------------
| ListTagsForResource|
+--------------------+
||      TagList     ||
|+--------+---------+|
||   Key  |  Value  ||
|+--------+---------+|
||  Tag1  |  tags1  ||
||  Tag2  |  tags2  ||
|+--------+---------+|

Implements #3492

[phinze]

LGTM!

[ghost]

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