feat: dynamodb plain keys

[RianFuro]

With this pull request, you can provide a plain key-value-pair as dynamodb hash- or sortKey. This allows for either providing static values, or provide arbitrary resolutions through vtl that are not covered by the given shortcuts.

I used the following schema, for brevity's sake:

name: 'key-name'
value: 'static value'

However I'm also up for calling them attributeName and attributeValue respectively, to keep more in line with the CloudFormation schema declaration.

TODO:

• [ ] Update documentation
• [ ] Probably include the new format in an integration test?