The copy command should allow tag to be specified when copying to-repo

**Describe the problem/challenge you have**
[A description of the current challenge that you are experiencing.]

Currently I can't specify a tag when using `imgpkg copy -i example/image:tag --to-repo repo.example.com/my/image` so I need to add the tag after looking up the digest and applying it manually

**Describe the solution you'd like**
[A clear and concise description of what you want to happen. If applicable a visual representation of the UX (ex: new CLI argument name, the behavior expected).]

Either support a `:tag` suffix for the `--to-repo` arg or have a separate `--tag` option

**Anything else you would like to add:**
[Additional information that will assist in solving the issue.]

Also the error message when I included the tag isn't the most helpful:

```
Error: Building import repository ref: repository can only contain the runes `abcdefghijklmnopqrstuvwxyz0123456789_-./`: trisberg/my-image:my-tag
```

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