Emit header in MsgUpdateClient events (#8624)

* emit header in update client msg

* update CHANGELOG

* update spec

* fix nil header bug

* use JSON encoding for emitting header

* Update x/ibc/core/spec/06_events.md

* use proto for encoding

* add tests

* encode to hex before emitting header in event

* add comment addressing reasoning for hex encoding

* Update CHANGELOG.md

Co-authored-by: Federico Kunze <31522760+fedekunze@users.noreply.github.com>

Co-authored-by: Alessio Treglia <alessio@tendermint.com>
Co-authored-by: Federico Kunze <31522760+fedekunze@users.noreply.github.com>

x/ibc/core/02-client/types/encoding_test.go

@@ -2,13 +2,13 @@ package types_test
 
 import (
 	"github.com/line/lbm-sdk/x/ibc/core/02-client/types"
-	ibctmtypes "github.com/line/lbm-sdk/x/ibc/light-clients/99-ostracon/types"
+	ibcoctypes "github.com/line/lbm-sdk/x/ibc/light-clients/99-ostracon/types"
 )
 
 func (suite *TypesTestSuite) TestMarshalHeader() {
 
 	cdc := suite.chainA.App.AppCodec()
-	h := &ibctmtypes.Header{
+	h := &ibcoctypes.Header{
 		TrustedHeight: types.NewHeight(4, 100),
 	}