Improve usability of Project & Service objects

[milas]

Description

The Project and Service objects returned by this library often
confuse users because certain fields are not always initialized.

For example, CustomLabels (a map[string]string) will be nil
on each Service.

Potential Solutions

• Add NewProject and NewService constructors that initialize
  all fields consistently
• Add helper methods, e.g. AddCustomLabel that internally handle
  initialization lazily

Related Issues

docker/compose#9808

[milas]

cc @glours I know we talked about this before around another Compose PR, so you might remember some other cases we should track here

[glours]

Users should add new elements to this kind of Map with the helper Add function that checks if the map is empty or not before adding a new value.

Anyway we can create a generic function for that as Golang 1.19 is out.
Golang 1.17 is not supported anymore so we may assume downstream projects to use at least Golang v1.18.x

[RiskyFeryansyahP]

Hello guys, sorry for joining the discussion here 🙏 .

how about on LoadService we add the process to initialize the CustomLabel to the empty value of map[string]string instead of leaving it nil ?
because most who use this compose-go library call it in loader.Load(). so when loader.Load() return the Project type the value of Service.CustomLabel not nil but already initialize.