rust(feat): Adds FlowDescriptor and FlowBuilder to improve performance

[tsift]

Creates two new public structs, the FlowDescriptor and the FlowBuilder. These are intended to abstract the complete FlowConfig and ChannelConfig to the minimum required for ingesting data once registered to Sift.

Internally, the FlowDescriptor will have an index map of some key that maps to the index of that channel within the flow; it will also hold the data type of that field to improve validation while building a flow.

The FlowDescriptor can be created manually, or by converting a FlowConfig. Here, we are using strings as the "key", though the key can be arbitrary, so long as it is unique per channel. For maximum performance, the returned indices for each registered channel should then be used when setting the value in order to avoid a hash lookup when building the flow below:

let mut flow_descriptor = FlowDescriptor::new("ingestion_config_id", "my_flow_name");
let my_channel_idx = flow_descriptor.add("my_channel_key", ChannelDataType::String);
let my_other_channel_idx = flow_descriptor.add("my_other_channel_key", ChannelDataType::Uint64);

let descriptor = FlowDescriptor::try_from((INGESTION_CONFIG_ID, flow)).unwrap();

Then to build a flow using the FlowBuilder, you can either use the "key" to set the value, or the index returned when registering the channel in the FlowDescriptor:

let mut flow_builder = FlowBuilder::new(&mut flow_descriptor);
flow_builder.set(my_channel_idx, "my_value".to_string());
flow_builder.set_with_key("my_other_channel_key", 123_u64);