Lixia (Sylvia) Lei

**Describe the bug** Image import from `releases-docker.jfrog.io` is stuck at running state and never finish **To Reproduce** Steps to reproduce the behavior: ```bash $ az acr import -n anyregistry --source...

Sub-task of #102

Currently, when [`Concurrency`](https://github.com/oras-project/oras-go/blob/main/copy.go#L148) is not specified, its value will be considered as infinity. However, this can be insecure. We should use a default value instead of infinity.

Since `oras.Resolve` and `oras.Copy` may cache content, they should have a `MaxMetadataBytes` option so that the memory won't be blew up.

Tag methods like [`oras.Tag`](https://github.com/oras-project/oras-go/blob/main/content.go#L30) and [`TagReference`](https://github.com/oras-project/oras-go/blob/2946dfcef0845c9cdf1b0872fe563d99d2c9beac/registry/repository.go#L90) should support multi-tagging.

We can introduce go-routines to speed up the Extended Copy at [here](https://github.com/oras-project/oras-go/blob/main/extendedcopy.go#L67-L72).

Make godoc easier to read. e.g. https://pkg.go.dev/oras.land/oras-go/v2@main#pkg-variables

Resolves #312 Signed-off-by: Lixia (Sylvia) Lei

As per the latest [update ](https://github.com/opencontainers/image-spec/pull/967)on the image-spec, the `artifactType` field will be omitted if empty. Hence, after the change is released, we can deprecate the default artifact type `application/vnd.unknown.artifact.v1`...