fix: add missing preconditions and update samples

[JesseLovelace]

Adds BlobTargetOption.[meta]generationmatch(long) and updates the samples to use it

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[BenWhitehead]

Thanks for adding this needed overload! One spelling that needs to be fixed.

Do we also want to add the corresponding [meta]generationNotMatch overloads as well?

[sydney-munro]

I think one of these should be commented out as just an example rather than having the code overwrite the first precondition.

[JesseLovelace]

I think this makes more sense, because it will actually behave appropriately based on whether the object already exists or not. If it doesn't exist, the original precondition isn't overwritten

[JesseLovelace]

Do you think it would look better as

Storage.BlobTargetOption precondition;
if (storage.get(bucketName, objectName != null) {
    precondition = ...;
} else {
    precondition = ...;

?

[burkedavison]

Alternative that avoids the duplicate storage.get call:

Storage.BlobTargetOption precondition = 
  Optional.ofNullable(storage.get(bucketName, targetObjectName))
    .map(Storage::getGeneration)
    .map(Storage.BlobTargetOption::generationMatch)
    .orElse(Storage.BlobTargetOption.doesNotExist())

[sydney-munro]

I think @JesseLovelace suggestion here is the most readable. If we wanted to avoid the duplicate call could we could save the object retrieved.