[Serving] Add flow topology support

[yaronha]

• add support for flows/DAG in serving runtime
• enhance the graph, support streams, multi-function, ..
• work with Storey as async runtime engine

[Hedingber]

Looks good
The main thing I have to say is that without very high test coverage no one but you will be able to touch that code.
This PR does add very nice coverage, It was hard for me to follow and verify exactly what covered and what's not, if there is something you know that is not, please add
Also I highly suggest system tests to the whole serving thing (doesn't have to be part of this PR, but we must add it at some point.

[Hedingber]

Suggested change

	raise ValueError(f"unsupported topology {topology}, use 'router' or 'flow'")
	raise mlrun.errors.MLRunInvalidArgumentError(f"unsupported topology {topology}, use 'router' or 'flow'")

This is repeating, please replace in other places as well

[yaronha]

done

[Hedingber]

• Change graphviz requirement to ~=0.16.0
• you changed ValueError only in 2 places, but you have many other places in the code you're adding it, replace to mlrun.errors...
• you wanted to rename self._error_stream no ?
• you change g to graph in _add_graphviz_flow but not in _add_graphviz_router
• I think that just like you changed > to $queue you should change * to something more meaningful
• If you've left the flow.plot everywhere, get it out of comment, change the path to be under from tests.conftest import results and add a comment above it "plotting the graph to help debugging"
• test_on_error seems duplicated, can be parameterized as well, also I would move the tests that are parameterized to a different file

[yaronha]

@Hedingber

• i can lock graphviz but u can see they have a ver every month
• re the * for router its different, its a prefix (e.g. *Ensemble) so cant change it to a name, people can always place the class as first arg instead