[dag] rename reliable_broadcast.rs to rb_handler.rs

[ibalajiarun]

Description

Since reliable broadcast sender is moved to its own crate, we now only handle the receiver side of the broadcast in DAG. Hence, renaming the module appropriately

Test Plan

Existing tests

[ibalajiarun]

Current dependencies on/for this PR:

• main
  • PR [dag] Handle certified node in DAG Driver #9312
    • PR [dag] Integrate Order Rule with dag driver #9452
      • PR [dag] Integrate Dag Fetcher with Dag Driver #9453
        • PR [dag] rename reliable_broadcast.rs to rb_handler.rs #9454 👈
          • PR [dag] bootstrap logic #9455
            • PR [dag] network sender implementation #9456
              • PR [dag] e2e integration test #9457
                • PR [dag] Integrate dag fetcher with RB node handler #9585
                  • PR [dag][bugfix] fix bitmask for incomplete first round #9723
                  • PR [dag] preliminary state sync implementation #9724

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