Presigned urls don't work if user provided its own azure credentials and init might break

[moritzraho]

Expected Behaviour

• files.init({ azure: { <with SAS urls> } } should throw a clear error when using files.generatePresignURL as there is no way to generate pre signed urls without valid access to the tvm or storage account credentials
• files.init({ azure: { <with storage account credentials> } }) should be able to generate presigned url using its own storage account credentials in the same way the TVM would do it when initializing the files sdk with OpenWhisk credentials

Actual Behaviour

• files.init({ azure: { <with SAS urls> } }) in a runtime action: initializes the tvm while it shouldn't and when using files.generatePresignURL fails with a cryptic error message as it attempts to generate a presign url using the TVM from a blob not managed by the TVM storage account
• files.init({ azure: { <with SAS urls> } }) outside a runtime action: breaks directly on init as it attempts to init the TVM without any OpenWhisk credentials

Same for files.init({azure: { <with storage account credentials>}}) while in that case we should be able to support pre-signed urls

[aiojbot]

JIRA issue created: https://jira.corp.adobe.com/browse/ACNA-896