Support attribute modifiers like `if_not_exists` in ddb sink updateExpression

[petermyers]

would be also nice to have an option to use functions such as if_not_exists(path, value) too

UpdateExpression: "SET #attr = if_not_exists(#attr, :val)"

Very useful for fields such as createdOn etc.

Originally posted by @vsnig in #432 (comment)