exim: add subpackages, for lookups, scripts and utils.

[patdowney]

This PR adds sub-packages that are present in the alpine package (https://gitlab.alpinelinux.org/alpine/aports/-/blob/master/community/exim/APKBUILD) that weren't in the Wolfi package. Specifically it adds sub-packages for lookups (postgres, mysql and others) as well as packaging the exim-utils and exim-scripts too. Additionally, as also present in the alpine package, it adds a pre-install script to configure the exim user upon package installation.

Fixes:

• https://github.com/wolfi-dev/os/issues/29568

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For new package PRs only

• [ ] This PR is marked as fixing a pre-existing package request bug
  • [x] Alternatively, the PR is marked as related to a pre-existing package request bug, such as a dependency
• [x] REQUIRED - The package is available under an OSI-approved or FSF-approved license
• [x] REQUIRED - The version of the package is still receiving security updates
• [ ] This PR links to the upstream project's support policy (e.g. endoflife.date)