feat: compress entire blobs protocol using zstd or lz4

[rklaehn]

Per-file compression was implemented in #93 , but per-file compression has various downsides.

1. it requires to copy the files, so the sendme send temp dir might become huge
2. compression of multiple small files won't be very efficient

A better approach would be to optionally compress the entire protocol under a different ALPN, using zstd or maybe lz4 since lz4 has a pure rust impl.

This is not yet possible in blobs, but will be possible once n0-computer/iroh-blobs#147 is merged.