AppSync: support full primary key in mapping template for DynamoDB GetItem

Currently, `GetItem` only supports specifying the partition key.

### Use Case

Support schemas which have partition and sort keys.

### Proposed Solution

Have an object model to describe the primary key.

### Other

* [X] :wave: I may be able to implement this feature request
* [ ] :warning: This feature might incur a breaking change

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This is a :rocket: Feature Request

Start by tracing how AppSync DynamoDB GetItem mapping templates currently represent the partition key. Investigate the existing schema and primary-key handling before deciding how the proposed object model should fit. Done means schemas with both partition and sort keys can generate a working GetItem mapping template, with existing partition-key behavior preserved.

Written by the indexing model from the issue text.