Expand return types of key generators

PK and SK values can be any _scalar_ value, not just strings.

Therefore change return type of PK, SK, and GSI PK + SK, generator functions to `NativeScalarAttributeValue`

Thanks to @ghbuck for issue #2

documentation/Entities.md

@@ -9,8 +9,8 @@ Each _Entity_ object must satisfy the [`Entity` interface](https://symphoniaclou
 ```typescript
 interface Entity<TItem extends TPKSource & TSKSource, TPKSource, TSKSource> {
   type: string
-  pk(source: TPKSource): string
-  sk(source: TSKSource): string
+  pk(source: TPKSource): NativeScalarAttributeValue
+  sk(source: TSKSource): NativeScalarAttributeValue
   convertToDynamoFormat?: EntityFormatter<TItem>
   parse: EntityParser<TItem>
   gsis?: Record<string, GsiGenerators>
@@ -86,7 +86,7 @@ This occurs during many operations, including `put` and `get`.
 
 > If your table only has a Partition Key, and doesn't have a Sort Key, see the section _PK-only Entities_ below.
 
-Each of `pk()` and `sk()` is passed an argument of type `TPKSource` or `TSKSource`, as defined earlier, and should return a string.
+Each of `pk()` and `sk()` is passed an argument of type `TPKSource` or `TSKSource`, as defined earlier, and should return a "scalar" value (specifically [`NativeScalarAttributeValue`](https://docs.aws.amazon.com/AWSJavaScriptSDK/v3/latest/Package/-aws-sdk-util-dynamodb/TypeAlias/NativeScalarAttributeValue/)).
 
 Let's go back to our example of `Sheep` from earlier. Let's say we have a particular sheep object that is internally represented as follows:
 

src/lib/entities.ts

@@ -1,4 +1,4 @@
-import { NativeAttributeValue } from '@aws-sdk/util-dynamodb/dist-types/models'
+import { NativeAttributeValue, NativeScalarAttributeValue } from '@aws-sdk/util-dynamodb/dist-types/models'
 
 export type DynamoDBValues = Record<string, NativeAttributeValue>
 
@@ -76,7 +76,7 @@ export interface Entity<TItem extends TPKSource & TSKSource, TPKSource, TSKSourc
    * often be some kind of encoding of one more fields
    * @param source
    */
-  pk(source: TPKSource): string
+  pk(source: TPKSource): NativeScalarAttributeValue
 
   /**
    * Given a subset of the fields of the entity type, generate the sort key value for the stored version of the object.
@@ -87,7 +87,7 @@ export interface Entity<TItem extends TPKSource & TSKSource, TPKSource, TSKSourc
    * often be some kind of encoding of one more fields
    * @param source
    */
-  sk(source: TSKSource): string
+  sk(source: TSKSource): NativeScalarAttributeValue
 
   /**
    * A map of GSI ID to generator functions, similar to pk() and sk()
@@ -104,7 +104,7 @@ export type PKOnlyEntity<TItem extends TPKSource, TPKSource> = Omit<Entity<TItem
 
 export interface GsiGenerators {
   // ToMaybe - better types for these?
-  pk(source: unknown): string
+  pk(source: unknown): NativeScalarAttributeValue
 
-  sk?(source: unknown): string
+  sk?(source: unknown): NativeScalarAttributeValue
 }