rpc: simplify the encoding of interface-typed arguments in JSON (#7600)

Add package jsontypes that implements a subset of the custom libs/json 
package. Specifically it handles encoding and decoding of interface types
wrapped in "tagged" JSON objects. It omits the deep reflection on arbitrary
types, preserving only the handling of type tags wrapper encoding.

- Register interface types (Evidence, PubKey, PrivKey) for tagged encoding.
- Update the existing implementations to satisfy the type.
- Register those types with the jsontypes registry.
- Add string tags to 64-bit integer fields where needed.
- Add marshalers to structs that export interface-typed fields.

crypto/sr25519/encoding.go

@@ -1,6 +1,9 @@
 package sr25519
 
-import tmjson "github.com/tendermint/tendermint/libs/json"
+import (
+	"github.com/tendermint/tendermint/internal/jsontypes"
+	tmjson "github.com/tendermint/tendermint/libs/json"
+)
 
 const (
 	PrivKeyName = "tendermint/PrivKeySr25519"
@@ -10,4 +13,7 @@ const (
 func init() {
 	tmjson.RegisterType(PubKey{}, PubKeyName)
 	tmjson.RegisterType(PrivKey{}, PrivKeyName)
+
+	jsontypes.MustRegister(PubKey{})
+	jsontypes.MustRegister(PrivKey{})
 }

crypto/sr25519/privkey.go

@@ -29,6 +29,9 @@ type PrivKey struct {
 	kp  *sr25519.KeyPair
 }
 
+// TypeTag satisfies the jsontypes.Tagged interface.
+func (PrivKey) TypeTag() string { return PrivKeyName }
+
 // Bytes returns the byte-encoded PrivKey.
 func (privKey PrivKey) Bytes() []byte {
 	if privKey.kp == nil {

crypto/sr25519/pubkey.go

@@ -23,6 +23,9 @@ const (
 // PubKey implements crypto.PubKey.
 type PubKey []byte
 
+// TypeTag satisfies the jsontypes.Tagged interface.
+func (PubKey) TypeTag() string { return PubKeyName }
+
 // Address is the SHA256-20 of the raw pubkey bytes.
 func (pubKey PubKey) Address() crypto.Address {
 	if len(pubKey) != PubKeySize {