Add surpport for []uint16 in HashTreeRootWith method

[fearlessfe]

sszgen version: 0.1.3
system: macos with m chip
go version: go1.23.1 darwin/arm64

Below is the struct

type ClientInfoAndCapabilitiesPayload struct {
	ClientInfo []byte `ssz-max:"200"`
	DataRadius []byte `ssz-size:"32"`
	Capabilities []uint16 `ssz-max:"400"`
}

generated method, there is no AppendUint16 method. There are AppendUint8 AppendUint32 AppendUint64, why no AppendUint16?

// HashTreeRootWith ssz hashes the ClientInfoAndCapabilitiesPayload object with a hasher
func (c *ClientInfoAndCapabilitiesPayload) HashTreeRootWith(hh ssz.HashWalker) (err error) {
	indx := hh.Index()

	// Field (0) 'ClientInfo'
	{
		elemIndx := hh.Index()
		byteLen := uint64(len(c.ClientInfo))
		if byteLen > 200 {
			err = ssz.ErrIncorrectListSize
			return
		}
		hh.Append(c.ClientInfo)
		hh.MerkleizeWithMixin(elemIndx, byteLen, (200+31)/32)
	}

	// Field (1) 'DataRadius'
	if size := len(c.DataRadius); size != 32 {
		err = ssz.ErrBytesLengthFn("ClientInfoAndCapabilitiesPayload.DataRadius", size, 32)
		return
	}
	hh.PutBytes(c.DataRadius)

	// Field (2) 'Capabilities'
	{
		if size := len(c.Capabilities); size > 400 {
			err = ssz.ErrListTooBigFn("ClientInfoAndCapabilitiesPayload.Capabilities", size, 400)
			return
		}
		subIndx := hh.Index()
		for _, i := range c.Capabilities {
			hh.AppendUint16(i) // no AppendUint16
		}
		hh.FillUpTo32()
		numItems := uint64(len(c.Capabilities))
		hh.MerkleizeWithMixin(subIndx, numItems, ssz.CalculateLimit(400, numItems, 2))
	}

	hh.Merkleize(indx)
	return
}