provide json package alternative

[mathetake]

somewhat related to #12

[mathetake]

After TinyGo supports WASI, we could use https://github.com/vugu/vjson

[mathetake]

func (ctx context) OnVMStart(vmConfigurationSize int) bool {
	data, err := proxywasm.HostCallGetVMConfiguration(vmConfigurationSize)
	if err != nil {
		proxywasm.LogCritical("error reading vm configuration", err.Error())
	}

	var c map[string]interface{}
	if err := vjson.Unmarshal(data, &c); err != nil {
		panic(err)
	}

	proxywasm.LogInfo("vm config: \n", c["name"].(string))
	return true
}

verified that it works with WASI target:

[2020-09-14 17:44:43.371][1014652][info][wasm] [external/envoy/source/extensions/common/wasm/context.cc:997] wasm log my_root_id: vm config:
vm configuration