Remove OpenStruct from CLI/Arguments parser

[Earlopain]

Ruby 3.4 will warn on usage of ostruct, 3.5 will raise. Also see https://bugs.ruby-lang.org/issues/20309, ruby/ruby#10428

Something like this should be a good substitute, however since there seem to be no tests for CLI behaviour I'm not confident enough to PR myself:

# arguments.rb

Options = Struct.new(
  :time,
  :counter,
  :mode,
  :digest,
  :secret,
)

def default_options
  Options.new time: true, counter: 0, mode: :time
end

def to_h
  { time: options!.time, counter: options!.counter, digest: options!.digest, secret: options!.secret! }
end

# cli.rb

if options.mode == :time
  ROTP::TOTP.new(options.secret, options.to_h).now
elsif options.mode == :hmac
  ROTP::HOTP.new(options.secret, options.to_h).at options.counter
end