bmc_monitor_app: Adding systemd boot process duration

Expose BMC bootup process durations.
This bootup process is defined by systemd[1] and has been included in
Redfish schema[2].
Basically the implementation is copied from systemd-analyze[3].

Ref:
[1] https://www.freedesktop.org/software/systemd/man/bootup.html
[2] https://www.dmtf.org/sites/default/files/standards/documents/DSP0268_2023.1.pdf
    (6.65 ManagerDiagnosticData 1.2.0)
[3] https://github.com/systemd/systemd/blob/82b7bf8c1c8c6ded6f56b43998c803843a3b944b/src/analyze/analyze-time-data.c

Tested:
```
\# Check the duration calculated by boot-time-monitor
bmc:~# cat /usr/share/boot_time_monitor/bmc_durations.csv.completed
Firmware,0
Loader,0
Kernel,6125
InitRD,0
Userspace,371105

\# Manual install systemd-analyze to ensure the result is correct.
bmc:~# systemd-analyze
Startup finished in 6.125s (kernel) + 6min 11.105s (userspace) = 6min 17.231s
multi-user.target reached after 2min 48.715s in userspace.
```

Google-Bug-Id: 296530445
Change-Id: Ifa632a920e4d23085f25e6a38e00475218e907c8
Signed-off-by: Michael Shen <gpgpgp@google.com>