Issue https://github.com/grpc/grpc/issues/34304
Allows to disable systemd support,
as well as linking with libsystemd,
when it is not required.
The option has three possible values:
AUTO - Default, Will try to find libsystemd. If found, systemd support will be enabled.
ON - Enable systemd support. Build will fail if libsystemd is not found.
OFF - Disable systemd support.
Closes#34384
COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/34384 from kirpichik:cmake-use-systemd-option 96f5f4bd68
PiperOrigin-RevId: 621274356