imgpkg init Command

**Describe the problem/challenge you have**

When first creating a Bundle with `imgpkg`, I would like the ability to generate all the contents of the `.imgpkg` directory with a single command. I would like to use this command directly through `imgpkg` and not directly use an additional tool such as `kbld`.

**Describe the solution you'd like**

`imgpkg init -f /files` creates the ImagesLock and Bundle metadata config in the `.imgpkg` directory.

Where applicable, we should consider allowing the user to have options to fill in the properties of the bundle metadata file.

`imgpkg init -f /files --name my-bundle`

**Anything else you would like to add:**

The behavior in this case could be calling out to `kbld` to generate the ImagesLock file as a sensible default.

Read the existing imgpkg command handling and the kbld flow that generates an ImagesLock first; the issue names no files or tests. Define the init command's supported flags and verify that it creates both ImagesLock and Bundle metadata in the .imgpkg directory, including the requested bundle name when provided.

Written by the indexing model from the issue text.