dynamodb/attributevalue: Encode map[string]struct{} as StringSet, not just []string

[finnbear]

Is your feature request related to a problem? Please describe.
In Go, sets of type T are commonly stored as map[T]struct{} or map[T]bool (also common but wastes memory). Unlike []T (slice of T), maps make it much easier to check if a given item is present. However, the DynamoDB SDK only allows marshaling these values as maps.

Describe the solution you'd like
Struct members of type map[string|number]struct{} (and optionally map[string|number]bool) should be able to be encoded/decoded as DynamoDB StringSet|NumberSet. For clarity, if map[string|number]bool was supported, false items would not be encoded. All decoded items would be true.

Note: BinarySet should be considered as a possible 3rd type of set to encode/decode using a map

Describe alternatives you've considered
I have not considered any other ways to marshal into a DynamoDB set.

https://www.davidkaya.com/sets-in-golang/

Originally filed under V1: aws/aws-sdk-go#3408

https://github.com/guregu/dynamo supports this.

[github-actions]

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