Support presignUrls on public files or explicitely throw an error

[moritzraho]

Expected Behaviour

presignUrls for public files might be useful for handing over write or delete permissions, obviously read is not useful.
we do not support it for now but still don't throw an error, and the user receives a non working presign URL

Actual Behaviour

via TVM credentials:

• init
• write('public/hello', 'world')
• generatePresignURL('public/hello', { expiryInSeconds: 60, permissions: 'rwd' })
• go to generated url => file not available
  Reason for failure: TVM will generate against the private container: https://github.com/adobe/aio-tvm/blob/master/lib/impl/AzurePresignTvm.js#L51

via BYO credentials:

• init
• write('public/hello', 'world')
• generatePresignURL('public/hello', { expiryInSeconds: 60, permissions: 'rwd' })
• go to generated url => file not available
  Reason for failure: same points to private container in all cases https://github.com/adobe/aio-lib-files/blob/master/lib/impl/AzureBlobFiles.js#L551

[aiojbot]

JIRA issue created: https://jira.corp.adobe.com/browse/ACNA-987

[purplecabbage]

isn't it easier just to support it? I would just prevent it from throwing an error and move on

[moritzraho]

@purplecabbage yes, it's about generating the presign URL against the public container instead of the private one when the file starts with 'public/' prefix
This needs fixing in two places:

• TVM case: fix here - https://github.com/adobe/aio-tvm/blob/master/lib/impl/AzurePresignTvm.js#L51
• BYO case: fix here - https://github.com/adobe/aio-lib-files/blob/master/lib/impl/AzureBlobFiles.js#L551