MsgTransferResponse add sequence (backport #2377) (#2465)

* MsgTransferResponse add sequence (#2377)

## Description

Returns sequence from `sendTransfer`, and returns it with the `MsgTransferResponse`. This is not an API breaking change.

Retrieving the sequence at the time of creating the transfer is necessary in the packet forward middleware for correlation with multihop packet flows.

strangelove-ventures/packet-forward-middleware#33
strangelove-ventures/interchaintest#306

Closes #1969

---

- [x] Targeted PR against correct branch (see [CONTRIBUTING.md](https://github.com/cosmos/ibc-go/blob/master/CONTRIBUTING.md#pr-targeting))
- [x] Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
- [x] Code follows the [module structure standards](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules/structure.md).
- [x] Wrote unit and integration [tests](https://github.com/cosmos/ibc-go/blob/master/CONTRIBUTING.md#testing)
Existing test coverage exercises this new method due to the re-routing of `SendTransfer` through `SendPacketTransfer`
- [x] Updated relevant documentation (`docs/`) or specification (`x/<module>/spec/`)
- [x] Added relevant `godoc` [comments](https://blog.golang.org/godoc-documenting-go-code).
- [x] Added a relevant changelog entry to the `Unreleased` section in `CHANGELOG.md`
- [x] Re-reviewed `Files changed` in the Github PR explorer
- [x] Review `Codecov Report` in the comment section below once CI passes

(cherry picked from commit 3363917)

# Conflicts:
#	docs/ibc/proto-docs.md
#	modules/apps/transfer/keeper/msg_server.go
#	modules/apps/transfer/keeper/relay.go
#	modules/apps/transfer/types/tx.pb.go

* fix conflicts

Co-authored-by: Andrew Gouin <[email protected]>
Co-authored-by: crodriguezvega <[email protected]>

CHANGELOG.md

@@ -42,6 +42,8 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 ### State Machine Breaking
 
+* (transfer) [\#2377](https://github.com/cosmos/ibc-go/pull/2377) Adding `sequence` to `MsgTransferResponse`.
+
 ### Improvements
 
 ### Features