feat: Add sd_notify support

[provokateurin]

Closes #283

Allows a systemd service to use Type=notify, so it's only considered ready once all the listeners are up.
Additionally I also added sd_notify Reloading support, so systemd knows when stalwart is reloading the config due to an API call to the management API.
See https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html#Options and https://www.freedesktop.org/software/systemd/man/latest/sd_notify.html#Well-known%20assignments.

[CLAassistant]

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[provokateurin]

@mdecimus could you have a look at this? It's a pretty small but very useful feature.

[mdecimus]

This needs to be gated as not everybody uses systemd or Linux. Also, see https://github.com/stalwartlabs/stalwart/blob/main/CONTRIBUTING.md as some contributions can't be accepted at this moment.

[provokateurin]

Stalwart is currently not accepting external contributions, except for bug fixes and small, well-scoped changes

I would consider this change a very small and well-scoped change 🤔

IMO suppressing the errors functions well enough as gating, but I'll implement a proper check 👍

[provokateurin]

Now a check for systemd support is in place.

[provokateurin]

@mdecimus could you take another look?

[mdecimus]

Sorry, this can't be merged now. I suggest to run your own custom build in the meantime.