[gcloud] Support GCS preconditions for object write/delete/copy

[Amayyas]

Adds optional ifGenerationMatch and ifMetagenerationMatch parameters to the object write, delete, metadata-update and copy operations, enabling optimistic concurrency control as described at https://cloud.google.com/storage/docs/request-preconditions.

The parameter types mirror the existing ObjectGeneration API (objectGeneration is a String, metaGeneration is an int), so a value read from ObjectInfo.generation can be passed straight back as a precondition without conversion. An ifGenerationMatch of 0 (as the string '0') limits a write/copy to the case where the object does not yet exist.

The change is purely additive and non-breaking.

Fixes #184

[jonasfj]

for the record, I think https://pub.dev/packages/google_cloud_storage is the future, consider giving it a try, I think it has some of these features.

[github-actions]

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[Amayyas]

Thanks for the pointer — I hadn't come across google_cloud_storage.

Reading through it, it does look like the direction of travel, but it's still marked experimental (0.6.2, under labs.dart.dev, with a warning about a high expected rate of breaking changes). So for anyone currently on package:gcloud there isn't a stable migration path yet, and #184 was labelled contributions-welcome, which is what prompted me to pick it up.

Happy to go whichever way is most useful to you:

• land this in gcloud as a stopgap for existing users (it's additive — the only compatibility caveat is for anyone implementing the Bucket / Storage abstract classes, which the issue already anticipated), or
• close this and look at whether google_cloud_storage is missing preconditions on the object write/delete/copy paths, and contribute there instead.

Just let me know which you'd prefer.