evidence: json tags for DuplicateVoteEvidence (#4959)

## Description

Add json tags to duplicate evidence 

Closes: #4958

CHANGELOG_PENDING.md

@@ -31,6 +31,7 @@ Friendly reminder, we have a [bug bounty program](https://hackerone.com/tendermi
     - multisig: type `PubKeyMultisigThreshold` is now `PubKey`
   - [light] \#4946 Rename `lite2` pkg to `light`, the lite cmd has also been renamed to `light`. Remove `lite` implementation.
   - [rpc] \#4937 Return an error when `page` pagination param is 0 in `/validators`, `tx_search` (@melekes)
+  - [evidence] \#4959 Add json tags to `DuplicateVoteEvidence`
 
 - Apps
 

types/evidence.go

@@ -336,8 +336,8 @@ func RegisterMockEvidences(cdc *amino.Codec) {
 // DuplicateVoteEvidence contains evidence a validator signed two conflicting
 // votes.
 type DuplicateVoteEvidence struct {
-	VoteA *Vote
-	VoteB *Vote
+	VoteA *Vote `json:"vote_a"`
+	VoteB *Vote `json:"vote_b"`
 }
 
 var _ Evidence = &DuplicateVoteEvidence{}