Add ability to drop errors instead of going into `sync` mode

[ananthakumaran]

  location: [
    initial_call: {:proc_lib, :init_p, 5},
    current_stacktrace: [
      {Process, :sleep, 1, [file: ~c"lib/process.ex", line: 303]},
      {Sentry.Transport, :post_envelope_with_retries, 5,
       [file: ~c"lib/sentry/transport.ex", line: 41]},
      {Sentry.Client, :encode_and_send, 4,
       [file: ~c"lib/sentry/client.ex", line: 228]},
      {Sentry.Client, :send_event, 2,
       [file: ~c"lib/sentry/client.ex", line: 124]},
      {:logger_backend, :call_handlers, 3,
       [file: ~c"logger_backend.erl", line: 51]},
      {:proc_lib, :exit_p, 3, [file: ~c"proc_lib.erl", line: 261]}
    ]
  ],

We recently got into a situation where the logger handler turned into sync mode and got stuck for a really long time. We would prefer to drop the messages instead of going to sync mode (which makes the situation even worse). We do have a rate limit configured, but this is hard to get right because the rate limit is local to a single process, whereas sentry API side rate limits are global.