perf: MsgTypeUrl optimization (cosmos#9530)

## Description

Tiny optimization: no need to do string formatting to prefix a string with one character.

---

### Author Checklist

*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.*

I have...

- [x] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [x] added `!` to the type prefix if API or client breaking change
- [x] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [ ] provided a link to the relevant issue or specification
- [x] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules)
- [x] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [x] added a changelog entry to `CHANGELOG.md`
- [x] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [x] updated the relevant documentation or specification
- [x] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

*All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

(cherry picked from commit 17ba2eb)

types/tx_msg.go

@@ -85,3 +85,8 @@ type TxDecoder func(txBytes []byte) (Tx, error)
 
 // TxEncoder marshals transaction to bytes
 type TxEncoder func(tx Tx) ([]byte, error)
+
+// MsgTypeURL returns the TypeURL of a `sdk.Msg`.
+func MsgTypeURL(msg Msg) string {
+	return "/" + proto.MessageName(msg)
+}

types/tx_msg_test.go

@@ -29,3 +29,7 @@ func (s *testMsgSuite) TestMsg() {
 	s.Require().Nil(msg.ValidateBasic())
 	s.Require().NotPanics(func() { msg.GetSignBytes() })
 }
+
+func (s *testMsgSuite) TestMsgTypeURL() {
+	s.Require().Equal("/testdata.TestMsg", sdk.MsgTypeURL(new(testdata.TestMsg)))
+}