Feature Request: Allow `--empty-oci-base` with `crane mutate`

**Description**
Currently creating an image with an empty OCI base and setting labels requires two separate commands: `crane append --oci-empty-base` followed by `crane mutate`. This can be streamlined by allowing the `--oci-empty-base` flag to be used directly with `crane mutate`.

**Current Workflow**

1. `crane append --oci-empty-base --new_layer image.tar --new_tag registry.example.com//:`
2. `crane mutate registry.example.com//: --label =....`

**Purposed Workflow**
1. `crane mutate --oci-empty-base --append image.tar registry.example.com//: --label =....`

I'm would like to contribute to this effort and have a working sample of crane mutate with the `--oci-empty-base` flag added.

**Benefits**
- reduced number of registry interactions
- reduced number of commands to produce an oci artifact

Start with the crane mutate command entry point and compare it with crane append's existing --oci-empty-base workflow. Done means mutate accepts the empty-base option together with --append and produces the requested OCI artifact with labels in one command; add or run relevant CLI coverage if available.

Written by the indexing model from the issue text.