feat: Add systemd journal logging capabilities to signstar-sign

Currently the output when viewed via journalctl --user --follow -t signstar-sign -o verbose looks like this (internal properties starting with _ are omitted for brevity):

Wed 2025-05-14 12:59:26.448237 CEST [s=0831b4b8f10141138b479b0bef946973;i=11281a2;b=05e3c19180394f1795fdf5d225518b8a;m=20c50ec73;t=6351671495673;x=2d3ee3f40974514f]
    PRIORITY=3
    SYSLOG_IDENTIFIER=signstar-sign
    CODE_FILE=signstar-sign/src/main.rs
    CODE_MODULE=signstar_sign
    CODE_LINE=141
    TARGET=signstar_sign
    VERSION=0.1.0
    MESSAGE=Processing signing request failed: SigningRequest(
                RequestDeserialization(
                    Error("EOF while parsing a value", line: 1, column: 0),
                ),
            )
    ERROR=Signing request error: Could not deserialize request: EOF while parsing a value at line 1 column 0
    SYSLOG_PID=123487

Additionally, in case the systemd journal is not available log to stderr via simplelog.

See: #176

I've also implemented configurable logging levels via the -v switch.