Add default_option method to Core::Configuration

[GustavoCaso]

What does this PR do?

This PR adds the default_option? to Core::Configuration

Motivation

I will port AppSec (ASM) configuration to use the one in Core::Configuration. The one key feature that the ASM configuration has that the core configuration has is the ability to check if a value has been set or if it is using the default value.

Here is the code ASM:

dd-trace-rb/lib/datadog/appsec/configuration.rb

Lines 88 to 90 in f7d1e5e

	def default_setting?(setting)
	settings.send(:default?, setting)
	end

dd-trace-rb/lib/datadog/appsec/remote.rb

Lines 107 to 109 in cbbd749

	def remote_features_enabled?
	Datadog::AppSec.send(:default_setting?, :ruleset)
	end

Once the code is merged, we can migrate the ASM configuration to use the core configuration

Additional Notes

How to test the change?

[GustavoCaso]

Close in favour of #2920