Kotlin
Add more flexibility to BinaryFormat.encodeToByteArray
What is your use-case and why do you need this feature?
I want to append additional data to the byte array generated by encodeToByteArray without having to copy all data into a larger array unnecessarily. I need to send this ByteArray over websockets but not all data that needs to be sent can be serialized.
Describe the solution you'd like
Option 1
Add additional parameters to encodeToByteArray to request empty space at the beginning and end of the array the this function instantiates.
Option 2
Receive a lambda that received the required space and returns a ByteArray on which to write serialized data and the starting index.
Normally, I would say you should use slices for that, but the .slice function copies an array.
The options given don't really work as encodeToByteArray doesn't actually know the needed array size ahead of time (due to the architecture of the library). It creates a dynamically sized byte buffer (e.g. ByteArrayOutputStream / ByteChannel) and writes to that before returning a byte buffer afterwards.
As such the ideal solution would be a to expose the serialization to a byte appendable type (Appendable for bytes or ByteArrayOutputStream etc.). There is no standard multiplatform type for this, and you really want to use a standard type (and a encodeToByteAppendable etc.).
While kotlinx.io provides such primitives this is not a dependency of the core serialization library, so a standard function to provide access is tricky.
