Add brcm cast plugin

[ummarfarooq-swe]

Adding broadcom CAST network plugin

What were the changes?
Broadcom's Congestion Aware Sprayed Traffic
(CAST) feature is implemented using CAST net plugin

Why were the changes made?
An implementation of QP Load Balance (LB) Scheduling that provides performance improvements over typical QP Load Balancing._

How was the outcome achieved?
The idea is to use Round Trip Time (RTT) measurements to schedule more traffic
over the QPs that are experiencing the least congestion.

Additional Documentation:
Refer README.md

Approval Checklist

Do not approve until these items are satisfied.

• Verify the CHANGELOG has been updated, if
  • there are any NCCL API version changes,
  • any changes impact library users, and/or
  • any changes impact any other ROCm library.

[gaoikawa]

Hi @ummarfarooq-swe - The PR process for RCCL has changed recently - please re-open this PR on the https://github.com/ROCm/rocm-systems repo.

Not required, but helpful: 

• prefix the PR with '[rccl]'
• apply the tag 'project: rccl'
• Add 'RCCL-Reviewers' to the PR (if you're able to.)

Thanks,
Gary