core: varlink enum for io.systemd.Unit interface (#40972)

Convert string fields to varlink enums in io.systemd.Unit

Following
https://github.com/systemd/systemd/pull/39391#discussion_r2489599449,
convert all configuration setting fields in the io.systemd.Unit varlink
interface from bare SD_VARLINK_STRING to proper enum types, adding type
safety to the IDL.

This converts ~30 fields across ExecContext, CGroupContext, and
UnitContext, adding 25 new varlink enum types.

Weak compatibility breakage (per
https://github.com/systemd/systemd/pull/40972#issuecomment-4222294318):
Varlink enum identifiers cannot contain - or +, so affected values are
underscorified on the wire. For example, "tty-force" becomes tty_force,
"kmsg+console" becomes kmsg_console.

The full list of affected values:
```
  - ExecInputType: tty-force, tty-fail
  - ExecOutputType: kmsg+console, journal+console
  - ProtectHome: read-only
  - CGroupController: bpf-firewall, bpf-devices, bpf-foreign, bpf-socket-bind, bpf-restrict-network-interfaces, bpf-bind-network-interface
  - CollectMode: inactive-or-failed
  - EmergencyAction: exit-force, reboot-force, reboot-immediate, poweroff-force, poweroff-immediate, soft-reboot, soft-reboot-force, kexec-force, halt-force, halt-immediate
  - JobMode: replace-irreversibly, ignore-dependencies, ignore-requirements, restart-dependencies
```