Support v4 Multi-Bucket Application Keys

[johnmaguire]

Fixes #27 with the ultimate goal of solving restic/restic#5741

A new b2.CreateKeyMultiBucket was added to avoid breaking the type signature of b2.CreateKey, with support for multi-bucket application keys.

b2.CreateKey continues to use the old API (similar to the terraform provider's usage of bucket_id vs bucket_ids) in order to support clients which do not support multi-bucket application keys.

Client.CreateKey accepts a new BucketIDs option and selects CreateKeyMultiBucket when len(bucketIDs) > 1.

References:

• Backblaze release notes: https://www.backblaze.com/apidocs/b2-authorize-account
• b2-sdk-python PR #538 (the porting reference):<Add support for multi-bucket application keys b2-sdk-python#538
• terraform-provider-b2 v0.12.1 (per-call v3/v4 dispatch): https://github.com/Backblaze/terraform-provider-b2/releases/tag/v0.12.1

[mlech-reef]

v4 b2_authorize_account response is parsed from the wrong location - restricted keys lose their scope

The v4 auth response nests key restrictions under apiInfo.storageApi.allowed:

• storageApi.allowed.buckets[] → { id, name }
• storageApi.allowed.namePrefix
• storageApi.allowed.capabilities

But StorageAPIInfo reads bucketIds/bucketNames/namePrefix at the storageApi top level, where v4 emits nothing. So for any bucket- or prefix-restricted key, B2.buckets and B2.pfx come back empty. The client never learns its own restriction, lists outside the allowed scope, and B2 rejects with 401:

=== RUN   TestListBucketContentsWithKey
    integration_test.go:1079: b2_list_file_names: 401:
--- FAIL: TestListBucketContentsWithKey (6.24s)

This also breaks the headline feature: a multi-bucket key authorizes but the client forgets which buckets it's scoped to.

Master (v3) returned bucketId/bucketName at the top level, so this only surfaced with the v4 migration. Unrestricted/master keys are unaffected (their allowed is empty), which is why most tests pass.

[mlech-reef]

https://www.backblaze.com/docs/cloud-storage-native-api-versions

[johnmaguire]

@mlech-reef Thanks for taking a look, and apologies on the miss - added an integration test for a multi-bucket key this time around and ran the suite.