Add the Apache 2.0 License as developers will be linking this into their
custom Wasm transforms.

Signed-off-by: Tyler Rockwood <rockwood@redpanda.com>

This package is a resizable buffer that tracks a reader and writer index
and is generally easier to use than bytes.Buffer for what we're trying
to do when reading/writing across the Wasm FFI boundary.

Signed-off-by: Tyler Rockwood <rockwood@redpanda.com>

This is the initial SDK for Wasm transforms.

We have an ABI that we expect from the broker, and the broker expects
from this SDK. We mostly pass buffers back and forth across the ABI
boundary, and in these buffers are Kafka wire format serialized records.

We operate on single records at a time so that we use as little memory as
possible within the Wasm runtime (there is very little overhead to the
Wasm function calls).

Lastly, the ABI is stubbed out on platforms other than Wasm so you can
test with the SDK without running it in Wasm.

Signed-off-by: Tyler Rockwood <rockwood@redpanda.com>

We add some simple documentation, and three examples.

Mirror: the simplest transform possible, just copy data to another
topic.

Regexp Filter: Shows off main() setup and environment variable
configuration

Transcoding: Hopefully something a little more real world without
pulling in Avro or some third party library.

Signed-off-by: Tyler Rockwood <rockwood@redpanda.com>