Add v2 model serving (and refactor serving as dir)

[yaronha]

this PR creates a new serving framework for models (but not limited running models), it comes with a special serving runtime (based on Nuclio) and follow common serving APIs (based on KFServing v2 and Triton), the serving framework supports complex topologies defined through a states graph (similar to AWS Step Functions), see more in the doc at mlrun/serving/README.md

[Hedingber]

@yaronha - A lot of repeating stuff:

• Naming conventions - it is NOT ok to give one char names to variables e can be event error exception env I can not and should not guess what's in there.
• characters don't cost money, please use full names function and not f fu or fn
• string formatting - use f"some string {some_variable}" instead of "some string {0}".format(some_variable)
• logging - use variables logging and not string formatting: logger.info("Some log message", some_variable=some_variable) instead of logger.info(f"Some log message: {some_variable}")
• Prefer using constants instead of the same reapeted hard coded string

[Hedingber]

@yaronha there are still plenty of unresolved comments from the previous review
Added some new comments.
Generally, there are tons of lines of codes here, very hard to understand the hierarchy between the different classes, please add some explanation in the PR description