[debug][zxdb-cli] Add versioned hello handshake

Implement a versioned hello message handshake between the CLI and the
daemon during startup. This ensures protocol compatibility and waits
for the daemon to be fully ready (connected to DAP) before the CLI
returns.

Test: fx test //scripts/debug/zxdb_cli
Change-Id: Ib6e299bd1bd48af5642c9948079b7c416a6a6964
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1594089
Fuchsia-Auto-Submit: Jacob Rutherford <jruthe@google.com>
Commit-Queue: Jacob Rutherford <jruthe@google.com>
Reviewed-by: Dorian Lee <dorianlee@google.com>