managers/apk: Support packages.repositories[].key

[nbuwe]

Not sure it it would have been better off as a feature request issue. Since I have a proposed patch, a draft PR seemed more expedient. Easy to file an issue retroactively if the procedure requires one.

This patch makes apk manager write packages.repositories[].key to /etc/apk/keys/$name if name is set.

Silently ignore entries that don't set both the name and the key so that existing configs that (ab)use (previously ignored) name as a comment don't break suddenly.

url and name+key are handled independently of each other, so you can combine them or keep them separate (one key for several repos) as necessary.

This allows for stuff like:

packages:
  manager: apk
  repositories:
    # custom repo and its key
    - url: https://localhost:8088/packages/user
      name: user-badc0ffe.rsa.pub
      key: |-
        -----BEGIN PUBLIC KEY-----
        ...

    # ci keys (common, fixed)
    - name: mergerequests@test-1057ca75.rsa.pub
      variants: [musl]
      key: |-
        -----BEGIN PUBLIC KEY-----
        ...

    - name: mergerequests@test-deadbeef.rsa.pub
      variants: [glibc]
      key: |-
        -----BEGIN PUBLIC KEY-----
        ...

    # ci ephemeral repo(s) for some MR(s)
    - url: https://ci/{{image.variant}}/mergerequests/MR-37/testing
    - url: https://ci/{{image.variant}}/mergerequests/MR-42/testing