Adds qBraid as a remote REST target in CUDA-Q

[TheGupta2012]

Summary

Adds qBraid as a remote REST target in CUDA-Q. Kernels authored via
cudaq.set_target("qbraid", ...) are lowered to OpenQASM 2.0, submitted to the qBraid v2
API, and results are parsed back into cudaq::sample_result — with zero dependency on
the external qbraid-sdk Python package.

What's included

• Target qbraid registered via runtime/cudaq/platform/default/rest/helpers/qbraid/
  • QbraidServerHelper.cpp — v2 API client (submit, poll, fetch, parse)
  • qbraid.yml — target config, declares machine and api_key arguments
  • CMake gated behind -DCUDAQ_ENABLE_QBRAID_BACKEND=ON
• Compiler pipeline qbraid-gate-set-mapping in lib/Optimizer/CodeGen/Passes.cpp
• QASM normalization — merges nvq++'s multi-creg output into a single creg
  qbraid__creg__[N];. Required to unblock hardware routes (Rigetti, IQM Garnet,
  IonQ-via-Braket); see helper comment for the per-backend failure modes fixed.
• Retry-with-backoff in processResults absorbs qBraid v2's COMPLETED→result-queryable
  race.

Testing

• C++ unit tests (unittests/backends/qbraid/): sample sync/async, observe sync/async,
  future serialization, api_key-via-target-arg, job failure, result retry, retry
  exhaustion
• Python tests (python/tests/backends/test_Qbraid.py): full Python-binding coverage
  against the mock
• Target tests (targettests/qbraid/*.cpp): 13 C++ lit tests shared with other QASM2
  backends

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