Managed Nested SubGraph with Scheduler Integration

[drslebedev]

Implement a structured approach to associate a domain-specific scheduler with each subgraph. This enables optimization strategies based on specific requirements, such as latency vs. throughput, or computational domains like FPGA and GPU execution. The scheduler should be instantiated with the nested graph and passed as a Block for higher-level usage.

Additionally, a plugin system for Schedulers, similar to the existing Block plugins, needs to be implemented.

This is a follow-up of the following issues/PRs:

• Add subgraphs with exported ports #479
• [8SP;6SP] Improve Nested/Sub-Graph Integration (formerly 'hier-blocks') opendigitizer#184