feat: make validator key injectable by application developers (backport #21608) (#21802)

Co-authored-by: Marko <marko@baricevic.me>
Co-authored-by: marbar3778 <marbar3778@yahoo.com>

server/types/app.go

@@ -5,6 +5,7 @@ import (
 	"io"
 
 	cmtproto "github.com/cometbft/cometbft/api/cometbft/types/v1"
+	cmtcrypto "github.com/cometbft/cometbft/crypto"
 	cmttypes "github.com/cometbft/cometbft/types"
 	"github.com/cosmos/gogoproto/grpc"
 
@@ -57,6 +58,9 @@ type (
 		// SnapshotManager return the snapshot manager
 		SnapshotManager() *snapshots.Manager
 
+		// ValidatorKeyProvider returns a function that generates a validator key
+		ValidatorKeyProvider() func() (cmtcrypto.PrivKey, error)
+
 		// Close is called in start cmd to gracefully cleanup resources.
 		// Must be safe to be called multiple times.
 		Close() error