Add pcbb_xoff parameters for dualtor-aa-56 topo and 50000_40m profile

[vivekverma-arista]

Description of PR

Summary: Add pcbb_xoff parameters for dualtor-aa-56 topo and 50000_40m profile for Arista-7260CX3. Fixes #165

Type of change

• [ ] Bug fix
• [ ] Testbed and Framework(new/improvement)
• [ ] Test case(new/improvement)

Back port request

• [ ] 202012
• [ ] 202205
• [ ] 202305
• [x] 202311
• [x] 202405

Approach

What is the motivation for this PR?

How did you do it?

How did you verify/test it?

Any platform specific information?

Supported testbed topology if it's a new test case?

Documentation

[vivekverma-arista]

@StormLiangMS and @XuChen-MSFT can you review this?

[ZhaohuiS]

@XuChen-MSFT could you please help review it?

[vivekverma-arista]

@StormLiangMS please help merge

[mssonicbld]

Cherry-pick PR to 202405: https://github.com/sonic-net/sonic-mgmt/pull/14484

[mssonicbld]

Cherry-pick PR to 202311: https://github.com/sonic-net/sonic-mgmt/pull/14485