Provide useful factories for `Change`

[ikhoon]

Change is a data class that only provides the default constructor.
It would be nice to offer various factories for users' convenience as like upstream Central Dogma did.
https://github.com/line/centraldogma/blob/ece893db888fbb5c4f1e28987b9aee10d78e5396/common/src/main/java/com/linecorp/centraldogma/common/Change.java#L65

[ikhoon]

@minwoox If you are looking for an issue, how about this? I think this might be worth it to ship with the initial version. 😆

[minwoox]

Thanks for the suggestion. Yeah, let me handle this issue. 😄