fix(pagination): allow operation input to specify pagination token

[icholy]

Description of changes:

The existing value at inputTokenName should not be overwritten if startingToken was not provided.

Use-Case:

I was surprised when the following DynamoDB scan didn't respect the ExclusiveStartKey property, but did respect the Limit property.

const paginator = paginateScan(
	{ client: documentClient },
	{
		TableName: 'my-table',
		ExclusiveStartKey: key,
		Limit: 1000,
	},
);

[kuhe]

released in https://www.npmjs.com/package/@smithy/core/v/3.1.0