feat!: allow passing down machine options from nontransparent/octetcounting parsers

[leodido]

Fixes #21

This PR introduces breaking changes because it changes the Parser interface.
As a consequence, before merging it I'm waiting for confirmation it works in a real scenario from the grafana/alloy folks (see issue). Also, it would cause v5 to get released.

What it does

By merging in this PR,
the nontransparent and octetcounting parsers can now specify which options to pass down to the underlying syslog RFC machine parsers (RFC3164 vs RFC5424).

It introduces a syslog.ParserOption to let you do so: syslog.WithMachineOptions(opts ...syslog.MachineOption).

Example:

nontransparent.NewParserRFC3164(
  syslog.WithMachineOptions(rfc3164.WithYear(rfc3164.CurrentYear{})),
  syslog.WithListener(accumulator),
).Parse(r)

[leodido]

Friendly ping @dehaansa :)