fix: ensure core block api runs through multilplexer native app (#4767)

## Overview

Due to the multiplexer copying start command functionality from the sdk,
I discovered that running native apps (for example, v4) will not expose
the core BlockAPI when through the multiplexer, i.e. the changes in
celestiaorg/cosmos-sdk#455 do not take effect
which spin up the core BlockAPI in-process.

The BlockAPI is however available right now when:
- Running embedded apps (for example, v3) through the multiplexer. 
- Running a v4 standalone app without the multiplexer.

The replication of start command code being distributed and copied
amongst different places is extremely error prone to manage, and quite
frankly, its spaghetti.

---------

Co-authored-by: chatton <[email protected]>

Author: damiannolan

docker/multiplexer.Dockerfile

@@ -16,7 +16,7 @@ ARG MAX_SQUARE_SIZE
 ARG UPGRADE_HEIGHT_DELAY
 # the docker registry used for the embedded v3 binary.
 ARG CELESTIA_APP_REPOSITORY=ghcr.io/celestiaorg/celestia-app-standalone
-ARG CELESTIA_VERSION="v3.10.0-rc0"
+ARG CELESTIA_VERSION="v3.10.0-rc2"
 
 # Stage 1: this base image contains already released binaries which can be embedded in the multiplexer.
 FROM ${CELESTIA_APP_REPOSITORY}:${CELESTIA_VERSION} AS base

multiplexer/abci/multiplexer.go

@@ -21,6 +21,7 @@ import (
 	pvm "github.com/cometbft/cometbft/privval"
 	"github.com/cometbft/cometbft/proxy"
 	"github.com/cometbft/cometbft/rpc/client/local"
+	coregrpc "github.com/cometbft/cometbft/rpc/grpc"
 	db "github.com/cosmos/cosmos-db"
 	"github.com/cosmos/cosmos-sdk/client"
 	"github.com/cosmos/cosmos-sdk/codec"
@@ -314,6 +315,18 @@ func (m *Multiplexer) startGRPCServer() (*grpc.Server, client.Context, error) {
 		return nil, m.clientContext, err
 	}
 
+	coreEnv, err := m.cmNode.ConfigureRPC()
+	if err != nil {
+		return nil, m.clientContext, err
+	}
+
+	blockAPI := coregrpc.NewBlockAPI(coreEnv)
+	coregrpc.RegisterBlockAPIServer(grpcSrv, blockAPI)
+
+	m.g.Go(func() error {
+		return blockAPI.StartNewBlockEventListener(m.ctx)
+	})
+
 	// Start the gRPC server in a goroutine. Note, the provided ctx will ensure
 	// that the server is gracefully shut down.
 	m.g.Go(func() error {