[FEATURE] [jai-workflow-core] Implement Split/Merge nodes for branching flows

[czelabueno]

Feature Title

Implement Split/Merge nodes for branching flows

Description

Be able to implement branching flow pattern. A Split node generate a branch which performs other nodes in parallel and separately, and then their results are aggregated at the end by a Merge node to generate a final answer.

Example:

Motivation

In many cases, the RAG flow system may have multiple parallel running branches, usually to increase the diversity of generated results.

Alternatives

No response

Additional Context

Modular RAG paper show the importance to support multiple flow pattern for complex use cases.