vyos.utils: T6244: add support for year timebase in seconds_to_human()

We only supported calculating seconds to weeks but not seconds to years. This
has been added.

Testcase:
from vyos.utils.convert import seconds_to_human

minute = 60
hour = minute * 60
day = hour * 24
week = day * 7
year = day * 365.25

for separator in ['', ' ', '-', '/']:

print(f'----- Using separator "{separator}" -----')
print(seconds_to_human(10, separator))
print(seconds_to_human(5* minute, separator))
print(seconds_to_human(3* hour, separator))
print(seconds_to_human(4* day, separator))
print(seconds_to_human(7 * week, separator))
print(seconds_to_human(10 * year, separator))
print(seconds_to_human(5*year + 4*week + 3*day + 2*hour + minute + 5, separator))
print()

cpo@LR1.wue3:~$ ./foo.py

• Using separator "" -----

10s
5m
3h
4d
7w
10y
5y4w3d2h1m5s

• Using separator " " -----

10s
5m
3h
4d
7w
10y
5y 4w 3d 2h 1m 5s

• Using separator "-" -----

10s
5m
3h
4d
7w
10y
5y-4w-3d-2h-1m-5s

• Using separator "/" -----

10s
5m
3h
4d
7w
10y
5y/4w/3d/2h/1m/5s