Support variables/constants in goverter:map (map custom)

[jmattheis]

Currently only methods are allowed in goverter:map. It would be useful to support variables / constants to, because they are sometimes already present. E.g.

package example

import "time"

// goverter:converter
type Converter interface {
    // goverter:map Value | time:Second
	Convert(Input) Output
}

type Input struct {
	Name string
}
type Output struct {
	Name  string
	Value time.Duration
}

this should normally convert Input#Name -> Output#Name and use the constant time.Second for Output#Value.

Please 👍 this issue if you like this functionality. If you have a specific use-case in mind, feel free to comment it.