add optional kbld platform selection option in App/Package specs

[cppforlife]

What this PR does / why we need it:

Context

We would like to use platform configuration on PackageBuild so that only images (not image indexes) are included in the built Package.

kbld's configuration will be carried over to running App/Package but will be ignored as older kapp-controller/kbld do not interpret it.

Does this PR introduce a user-facing change?

- Add optional kbld platform configuration

Additional Notes for your reviewer:

• I believe there needs to be a follow up commit in CLI to use updated kapp-controller package once new kapp-controller version is cut

Review Checklist:

• Follows the developer guidelines
• Relevant tests are added or updated
• Relevant docs in this repo added or updated
• Relevant carvel.dev docs added or updated in a separate PR and there's
  a link to that PR
• Code is at least as readable and maintainable as it was before this
  change

Additional documentation e.g., Proposal, usage docs, etc.:

Platform can be specified on App/Package CRs. This passes through value to underlying kbld's platform flag.

...
- kbld:
    paths: [".imgpkg/images.yml", "config/"]
    platform: "linux/amd64"
...

[praveenrewar]

LGTM! @joaopapereira (since you added the generated code) if the rest looks good to you, I will merge it.

[joaopapereira]

given that you approved it I do not think we need the back and forth 😄