Best effort to support ollama registry

[shizhMSFT]

Ollama registry registry.ollama.ai is a container registry with incomplete APIs (ollama/ollama#6510, ollama/ollama#5021, ollama/ollama#5298) and does not support auth (ollama/ollama#7244).

As tested, oras-go is able to fetch blobs but not able to fetch manifests since the ollama registry does not return Content-Length header. As the OCI distribution-spec states

A successful response SHOULD contain the size in bytes of the uploaded blob in the header Content-Length.

the presentation of Content-Length is not a MUST. Therefore, oras-go is allowed to support this scenario with security well assessed.

[shizhMSFT]

One solution may be falling back to GET if HEAD manifest is 405 not allowed.

Since the fallback behavior may not be expected, we probably need an option (default to false) for that.

[shizhMSFT]

Since ollama pull does not work with local registry well, let's put this issue in the backlog.

Here's the log with the distribution/distribution.

$ ollama pull --insecure localhost:5000/test/deepseek-r1:7b
pulling manifest
Error: http: no Location header in response