feat: add xml marshal for Int type

[ruojunm]

Description

feat: add xml marshal for Int type

Rationale

When client uses golang/encoding/xml marshaler, it can't marshal cosmos Int type. So this PR will solve this problem.

Example

func (s *intTestSuite) TestIntXmlMarshalRoundTrip() {
	u1 := math.NewInt(256)
	marshalResult, _ := xml.Marshal(u1)
	u2 := math.Int{}
	xml.Unmarshal(marshalResult, &u2)
	s.Require().Equal(u1, u2)
}

Changes

Notable changes:

• add xml marshal and unmarshal methods for Int type
• add ut