Push fails on Windows if filepath has drive in it

[sunnycarter]

blob is C:\Users\blah\test.json
The command failed with an unexpected error. Here is the traceback:
C does not exist.
Traceback (most recent call last):
File "C:\Users\sunnycarter.azure\cliextensions\aosm\oras\client.py", line 131, in push
return self.remote.push(*args, **kwargs)
File "C:\Users\sunnycarter.azure\cliextensions\aosm\oras\provider.py", line 647, in push
raise FileNotFoundError(f"{blob} does not exist.")

I believe this is because various places in the oras code split on ':' expecting this to be between the filepath and the media data. However, ':' is in the path on window too. I'll submit a PR to fix.

[sunnycarter]

Fix in https://github.com/oras-project/oras-py/pull/94/files