az storage azcopy blob: allow arbitrary arguments

### Extension name
storage-preview

### Description of issue

The `az storage azcopy blob` commands support only a limited number of the underlying `azcopy` flags for each command. Currently, to get past that you have to use `az storage azcopy run-command`, but then you lose the automatic connection string / SAS configuration.

### Proposal
Every `az storage azcopy blob` command takes an `--additional-flags` string flag that can be appended to the ones built by the CLI for advanced scenarios. For example, `--additional-flags "--put-md5 true"`.