Add issue template for oras-go

[FeynmanZhou]

Hi maintainers,

I am thinking it would be better to replicate the issue template from ORAS repo to oras-go since it helps issue creators to shape the issue content to a standard structure and added related labels automatically. It would be convenient for issue creators to provide enough context by following a content structure so that maintainer are easy to triage issues.

[dextrot]

Hi @FeynmanZhou
Can i submit a PR for this issue?