Add Fluent API

[rcoh]

Issue #, if available:

Description of changes: Adds a fluent API layer that is a little bit less flexible but more ergonomic for most use cases

    let new_table = client
        .create_table()
        .table_name("test-table")
        .key_schema(vec![KeySchemaElement::builder()
            .attribute_name("k")
            .key_type(KeyType::Hash)
            .build()])
        .attribute_definitions(vec![AttributeDefinition::builder()
            .attribute_name("k")
            .attribute_type(ScalarAttributeType::S)
            .build()])
        .provisioned_throughput(
            ProvisionedThroughput::builder()
                .write_capacity_units(10)
                .read_capacity_units(10)
                .build(),
        )
        .send().await?

I ended up making some other refactorings to enable creating cargo features.
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[davidbarsky]

Ship-it!