Fix multi-asic support to PFC config/show

[arista-hpandya]

What I did

Added multi-asic support to PFC show and config commands by:

• Adding namespace arg for show and config cmds for pfc
• Replacing test DB with JSON to support verification of multiple namespaces in unit test
• Adding unit tests for multi-asic behaviour
• Adding a test vector file for better test organization

This is a logical successor of #3057 and is a part of the larger efforts to add multi-asic functionality tracked in #15148

How I did it

I used the multi_asic helper functions from sonic_py_common and utilities_common to handle multiple namespaces

How to verify it

There were three stages of verification:

• Verify the single-asic and multi-asic unit tests for pfc pass
• Verify the behaviour on a single-asic linecard on a DUT
• Verify the behaviour on a multi-asic linecard on a DUT

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)

The namespace argument is the new addition to the pfc command.

Case 1: If namespace is specified in a multi-asic device
Behaviour: We will run the command only on the specified namespace
e.g: pfc show asymmetric --namespace asic0

Case 2: If no namespace is specified in a multi-asic device
Behaviour: We will run the command on all namespaces
e.g: pfc show priority

Case 3: If a namespace is specified in single-asic device (other than default '')
Behaviour: We will raise an invalid argument error
e.g: pfc show asymmetric --namespace asic0

Case 4: If no namespace is specified in single-asic device
Behaviour: We will run it on the only valid namespace - default namespace
e.g: pfc show priority

[kenneth-arista]

@vmittal-msft @wenyiz2021 @arlakshm FYI

[wenyiz2021]

result pasted in PR description LGTM

[wenyiz2021]

to be cherry pick in 202405, after #3057 is ported to 202405

[mssonicbld]

Cherry-pick PR to 202405: #3550