rpc: use encoding/json rather than tmjson (#7670)

The main change here is to use encoding/json to encode and decode RPC 
parameters, rather than the custom tmjson package. This includes:

- Update the HTTP POST handler parameter handling.
- Add field tags to 64-bit integer types to get string encoding (to match amino/tmjson).
- Add marshalers to struct types that mention interfaces.
- Inject wrappers to decode interface arguments in RPC handlers.

crypto/ed25519/ed25519.go

@@ -13,7 +13,6 @@ import (
 	"github.com/tendermint/tendermint/crypto"
 	"github.com/tendermint/tendermint/crypto/tmhash"
 	"github.com/tendermint/tendermint/internal/jsontypes"
-	tmjson "github.com/tendermint/tendermint/libs/json"
 )
 
 //-------------------------------------
@@ -57,9 +56,6 @@ const (
 )
 
 func init() {
-	tmjson.RegisterType(PubKey{}, PubKeyName)
-	tmjson.RegisterType(PrivKey{}, PrivKeyName)
-
 	jsontypes.MustRegister(PubKey{})
 	jsontypes.MustRegister(PrivKey{})
 }

crypto/secp256k1/secp256k1.go

@@ -11,7 +11,6 @@ import (
 	secp256k1 "github.com/btcsuite/btcd/btcec"
 	"github.com/tendermint/tendermint/crypto"
 	"github.com/tendermint/tendermint/internal/jsontypes"
-	tmjson "github.com/tendermint/tendermint/libs/json"
 
 	// necessary for Bitcoin address format
 	"golang.org/x/crypto/ripemd160" // nolint
@@ -27,9 +26,6 @@ const (
 )
 
 func init() {
-	tmjson.RegisterType(PubKey{}, PubKeyName)
-	tmjson.RegisterType(PrivKey{}, PrivKeyName)
-
 	jsontypes.MustRegister(PubKey{})
 	jsontypes.MustRegister(PrivKey{})
 }

crypto/sr25519/encoding.go

@@ -2,7 +2,6 @@ package sr25519
 
 import (
 	"github.com/tendermint/tendermint/internal/jsontypes"
-	tmjson "github.com/tendermint/tendermint/libs/json"
 )
 
 const (
@@ -11,9 +10,6 @@ const (
 )
 
 func init() {
-	tmjson.RegisterType(PubKey{}, PubKeyName)
-	tmjson.RegisterType(PrivKey{}, PrivKeyName)
-
 	jsontypes.MustRegister(PubKey{})
 	jsontypes.MustRegister(PrivKey{})
 }