Replace our serde/MsgPack encoding of enum LogMsg with protobuf

[emilk]

We want to replace this:

rerun/crates/store/re_log_types/src/lib.rs

Lines 257 to 279 in 7fe8240

	#[must_use]
	#[derive(Clone, Debug, PartialEq)] // `PartialEq` used for tests in another crate
	#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
	#[allow(clippy::large_enum_variant)]
	pub enum LogMsg {
	/// A new recording has begun.
	///
	/// Should usually be the first message sent.
	SetStoreInfo(SetStoreInfo),
	/// Log an entity using an [`ArrowMsg`].
	//
	// TODO(#6574): the store ID should be in the metadata here so we can remove the layer on top
	ArrowMsg(StoreId, ArrowMsg),
	/// Send after all messages in a blueprint to signal that the blueprint is complete.
	///
	/// This is so that the viewer can wait with activating the blueprint until it is
	/// fully transmitted. Showing a half-transmitted blueprint can cause confusion,
	/// and also lead to problems with space-view heuristics.
	BlueprintActivationCommand(BlueprintActivationCommand),
	}

with something protobuf, ideally inspired/overlapping with our gRPC datatypes.

That means our .rrd files become just a bunch of protobuffs concatted together, and will be very similar (or same) as the gRPC messages we send to/from our data platform