This PR adds the new debug config option:

config.debug = true # default is false

As of now, SDK related errors are only logged with their error messages.

For example, assume we have this before_send callback:

  config.before_send = lambda do |event, hint|
    foo
  end

The SDK only logs this

Event sending failed: undefined local variable or method `foo' for main:Object

This is inconvenient for our users to debug their customization on the SDK. It also makes determining/reporting SDK issues harder (see #1383 (comment) for example).

So with when the new config.debug option set with true, it will log SDK errors with backtrace, just like we normally treat errors in our applications.

Event sending failed: undefined local variable or method `foo' for main:Object
/Users/st0012/projects/sentry-ruby/sentry-rails/examples/rails-6.0/config/initializers/sentry.rb:10:in `block (2 levels) in <main>'
/Users/st0012/projects/sentry-ruby/sentry-ruby/lib/sentry/client.rb:78:in `send_event'
/Users/st0012/projects/sentry-ruby/sentry-ruby/lib/sentry/client.rb:102:in `block in dispatch_background_event'

Closes #1297