[BUG] Refactor add/remove connection api in TMAN designer backend #1462
Description
With the addition of many syntactic sugars to TEN graph, such as names, selector, reversed_connection, etc., the previous tman designer would flatten the graph and provide the flattened version to the frontend for display and modification.
Now, after a series of adaptations, the frontend can display complex graphs, but adding or deleting connections is not yet supported.
This requires the tman designer backend to modify the API protocol and be able to operate directly on the graph before flattening.
The request payload can be modified as following:
