Enhanced support for single-table design

This is somewhat related to the caveat mentioned in the TransactWriteItems section regarding multi-table operations not being supported.

For a single-table design this limitations also pops if there are multiple instances of TableContext using the same table name, and a need arises to write more than one record type in the same request:

let baseContext : TableContext<BaseSchema> = ...
let contextFoo : Context<Foo> =  baseContext.WithRecordType<Foo>()
let contextBar : Context<Bar> =  baseContext.WithRecordType<Bar>()

let fooItems : Foo list = ...
let barItems : Bar list = ...

async {
  // Separate DynamoDB requests are needed here
  let! _ = contextFoo.BatchPutItemsAsync fooItems
  let! _ = contextBar.BatchPutItemsAsync barItems
}

The most straightforward way of resolving the immediate problem would be to make the AttributeValue helper methods on RecordTemplate here public, and from there just use the SDK to make the necessary request.

If there is another approach that offers a solution that doesn't require working with the SDK directly, and maybe solves the problem more generally for transactions as well I'd be happy to take a look, just let me know.

Start with RecordTemplate.fs at lines 263-284 and the TransactWriteItems documentation section, then trace TableContext.WithRecordType and BatchPutItemsAsync. Define what API should support mixed record types in one request and whether the same approach should cover transactions; completion should address the single-table case without requiring callers to use the SDK directly.