Remove use of unsafe json-iterator / reflect2 libraries

[liggitt]

The etcd client currently depends on json-iterator via the client/v2 package:

etcd/client/v2/json.go

Lines 60 to 73 in ef1f71a

	// caseSensitiveJsonIterator returns a jsoniterator API that's configured to be
	// case-sensitive when unmarshalling, and otherwise compatible with
	// the encoding/json standard library.
	func caseSensitiveJsonIterator() jsoniter.API {
	config := jsoniter.Config{
	EscapeHTML: true,
	SortMapKeys: true,
	ValidateJsonRawMessage: true,
	CaseSensitive: true,
	}.Froze()
	// Force jsoniter to decode number to interface{} via int64/float64, if possible.
	config.RegisterExtension(&customNumberExtension{})
	return config
	}

This is still relied on by portions of the v3 libraries:

etcd/server/etcdserver/api/v2discovery/discovery.go

Line 34 in ef1f71a

"go.etcd.io/etcd/client/v2"

The approach taken by json-iterator and its reflect library is not stable (it pins to internal details of the stdlib reflect package), and requires maintenance per go release, which does not appear to be sustainable (golang/go#48238 (comment))

In addition to the maintenance aspect, it is hard to reason about the safety of the implementation (golang/go#48238 (comment))

Kubernetes has transitioned away from json-iterator and is using sigs.k8s.io/json - a library based on the stdlib json decoder that added in case-sensitivity and int-preserving behavior.

[hexfusion]

@liggitt thanks, I appreciate the detailed issue.

/assign

[hexfusion]

@chaochn47 PTAL