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:

• exim - external DB lookups support not working #29568

Pre-review Checklist

For new package PRs only

• This PR is marked as fixing a pre-existing package request bug
  • Alternatively, the PR is marked as related to a pre-existing package request bug, such as a dependency
• REQUIRED - The package is available under an OSI-approved or FSF-approved license
• 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)

[willdeane]

I've done some testing of this update as it fixes the issue I raised in #29568. I can confirm that the pgsql package works as expected. I've also checked that if you install the other lookup packages Exim does detect and load them on start-up, although I've not done any actual testing for the other lookup types:

/ # /usr/sbin/exim -bdf -d
Exim version 4.98 uid=0 gid=0 pid=104 D=f7715cfd
Support for: Content_Scanning crypteq Expand_dlfunc IPv6 move_frozen_messages OpenSSL TLS_resume DANE DKIM DNSSEC ESMTP_Limits ESMTP_Wellknown Event OCSP PIPECONNECT PRDR PROXY Queue_Ramp SPF TCP_Fast_Open
Lookups (built-in): lsearch wildlsearch nwildlsearch iplsearch dsearch passwd
Authenticators: cram_md5 dovecot plaintext spa tls
Routers: accept dnslookup ipliteral iplookup manualroute queryprogram redirect
Transports: appendfile/maildir autoreply lmtp pipe smtp
Malware: f-protd f-prot6d drweb aveserver fsecure kavdaemon sophie clamd mksd avast sock cmdline
Fixed never_users: 0
Configure owner: 0:0
Size of off_t: 8
Compiler: GCC [14.2.0]
Library version: Glibc: Compile: 2.40
                        Runtime: 2.40
Hints DB:
 Using tdb
Library version: OpenSSL: Compile: OpenSSL 3.3.2 3 Sep 2024
                          Runtime: OpenSSL 3.3.2 3 Sep 2024
                                 : built on: Tue Sep  3 16:14:00 2024 UTC
Library version: spf2: Compile: 1.2.11
                       Runtime: 1.2.11
Library version: PCRE2: Compile: 10.44
                        Runtime: 10.44 2024-06-07
Loading lookup modules from /usr/lib/exim/
Loaded "mysql.so" (1 lookup types)
Loaded "dnsdb.so" (1 lookup types)
Loaded "cdb.so" (1 lookup types)
Loaded "sqlite.so" (1 lookup types)
Loaded "dbmdb.so" (3 lookup types)
Loaded "pgsql.so" (1 lookup types)
Loaded 6 lookup modules
Total 16 lookups
Library version: CDB: Exim version 4.98
Library version: DBM: Exim version 4.98
Library version: DNSDB: Exim version 4.98
Library version: MySQL: Compile: 100808 10.8.8 [mariadb-10.8]
                        Runtime: 30401 3.4.1
                        Exim version 4.98
Library version: PostgreSQL: Exim version 4.98
Library version: SQLite: Compile: 3.45.1
                         Runtime: 3.45.1
                         Exim version 4.98

The pre-installs script to add the exim user and the addition of the Exim utility scripts are also very useful improvements.