Adding the default fallback service capability to multiplex processors as discussed after introducing the multiplex protocol in THRIFT-1915.

multiplexed_processor = Thrift::MultiplexedProcessor.new
multiplexed_processor.register_default(MyService::LegacyProcessor.new(handler))
multiplexed_processor.register_processor('MyService', MyService::Processor.new(handler))

This functionality is already present in Java, C++, Python, Go, Rust, Swift, Perl, Haxe, C GLib (but seem to be missing from NodeJS, .NET, PHP). It is also being tested by multi* family in cross-tests.

Additionally, I removed the separate group for Ruby from cross-tests, as with UUID support implemented a while ago it is not longer needed.

• Did you create an Apache Jira ticket? THRIFT-6015
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• Did you squash your changes to a single commit? (not required, but preferred)
• Did you do your best to avoid breaking changes? If one was needed, did you label the Jira ticket with "Breaking-Change"?
• If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources.