Add new field for HexNumerics

[madhr]

Add new field type HexNumeric that allows to store binary encoded fields as hex numeric fields.

It's related to this change:
#227
where it's now possible to treat binary encoded fields as Hex.
Since Hex is a string field, it's a bit inconvenient for fields that store numeric values like amounts (for instance DE55.9F02).
Introducing the HexNumeric allows to convert the binary encoded fields straight into a hex numeric field.

[alovak]

Hey, @madhr! Thanks for the PR. I don't think we need the suggested changes as you can get the same results using BCD encoding:

func TestHexNumeric(t *testing.T) {
	numeric := NewNumeric(&Spec{
		Length:      4,
		Description: "Field",
		Enc:         encoding.BCD,
		Pref:        prefix.BCD.Fixed,
	})

	numeric.SetValue(1234)

	packed, err := numeric.Pack()
	require.NoError(t, err)

	require.Equal(t, []byte{0x12, 0x34}, packed)

	numeric2 := NewNumeric(&Spec{
		Length:      4,
		Description: "Field",
		Enc:         encoding.BCD,
		Pref:        prefix.BCD.Fixed,
	})

	_, err = numeric2.Unpack(packed)
	require.NoError(t, err)

	require.Equal(t, int64(1234), numeric2.Value())
}

[alovak]

@madhr does the snippet I shared above work for you?

[madhr]

@madhr does the snippet I shared above work for you?

yes thanks!