Ability to use specific platform when generating ImagesLock

**Describe the problem/challenge you have**

kbld is often used to generate lockfiles that are passed to imgpkg to generate a bundle. When such a lockfile contains a multi-platform image index, the resulting imgpkg bundle will include all images in the index. This is great if you need to distribute the bundle to a lot of different platforms, but if you know the exact platform you're targeting then including all those other platform images in the bundle wastes a lot of space (Windows images in particular can be very large).

**Describe the solution you'd like**
kbld should be able to replace index references with specific images based on the desired platform (OS, architecture). That way, imgpkg won't bundle up unwanted images.

**Anything else you would like to add:**
Discussed in https://kubernetes.slack.com/archives/CH8KCCKA5/p1639157591175300

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Questions:
1. what should the UX be?
- command-line args (e.g. `--force-arch=X --force-platform=Y`)
- addition(s) to [`kind: Config`](https://carvel.dev/kbld/docs/latest/config/)
2. should `origins:` include the platform/architecture supplied by the user? (more generally, should we include the total command-line used?)
3. ...

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