1486: dynamo/attributevalue allow json tag encoding/decoding

[moogacs]

#1486 reads json tags on marshalling structs

[jasdel]

Thanks for taking the time to create this PR @moogacs I updated #1486 with information how to use alternative tags with the attributevalue package's (un)marshalers.

In addition, adding support for json struct tag name by default could be a breaking change for some users where serialization of structure field names would change, and no longer be compatible with their data stored in their table.

Lets use #1486 and #1494 for discussion of this issue.