Implement logger broadcasting

[unflxw]

This PR builds on top of the changes in #1355.

Docs need to be updated to recommend this over Rails.logger.broadcast_to: https://github.com/appsignal/appsignal-docs/pull/2315

---

Implement logger broadcasting

Implement broadcasting logs to other loggers, following the same
interface as #broadcast_to. Unlike #broadcast_to, however,
instead of forwarding each method call one-for-one, we rely on the
#add method which is expected of the logger interface.

As such, we should not run into the kinds of bugs that affect
ActiveSupport::BroadcastLogger when used alongside additional
logger features such as tagged logging, such as rails/rails#49745.

This enables customers to use both broadcast logging and tagged
logging with the AppSignal logger without running into
application-breaking bugs.

As a trade-off, unlike ActiveSupport::BroadcastLogger, our
broadcast implementation is sensitive to the order in which those
additional features are applied, expecting them to be applied on
top of the broadcast logger, and not on the underlying loggers:

 # Incorrect example
some_logger = ActiveSupport::TaggedLogging.new(
  Logger.new("/etc/app.log")
)

appsignal_logger = Appsignal::Logger.new("app")

 # Even if `some_logger` is a tagging-enabled logger,
 # this logger will not behave as a tagging-enabled logger.
Rails.logger = appsignal_logger.broadcast_to(some_logger)

 # Correct example
some_logger = Logger.new("/etc/app.log")
appsignal_logger = Appsignal::Logger.new("app")

appsignal_logger.broadcast_to(some_logger)
 # As the tagged logging is applied on the broadcast logger,
 # both logs sent to AppSignal and to `some_logger` will include
 # given tags, and `Rails.logger` will be a tagging-enabled logger.
Rails.logger = ActiveSupport::TaggedLogging.new(appsignal_logger)