feat: support `add_<field>` for integer fields

[lrstanley]

✨ Describe the feature you'd like

Ent integer fields which are immutable support integer addition. This is to allow incrementing the number without knowing the existing number ahead of time, and/or to cover the scenario of incrementing the number without being able to do it in a transaction and losing potential data.

Example:

// AddViewCount adds i to the "view_count" field.
func (pu *PostUpdate) AddViewCount(i int) *PostUpdate {
	pu.mutation.AddViewCount(i)
	return pu
}

⚠ If implemented, do you think this feature will be a breaking change to users?

No

References

• https://english.stackexchange.com/questions/55416/hypernym-for-increase-and-decrease