Fix multi-asic behaviour for watermarkstat

[bktsim-arista]

What I did

Added multi-asic support to watermarkstat, fixing watermark/persistent-watermark related commands. Previously, the following commands were not behaving correctly on multi-asic devices, as the '-n' namespace option was not available, and correct namespaces were not traversed on multi-asic devices.

• show buffer_pool watermark/persistent-watermark
• show headroom-pool watermark/persistent-watermark
• show priority-group persistent-watermark/watermark
• show queue persistent-watermark/watermark

This change fixes multi-asic behaviour of CLI commands that rely on watermarkstat as listed above.

This is a part of the set of changes being pushed for https://github.com/sonic-net/sonic-buildimage/issues/15148

How I did it

Added namespace option -n and used multi_asic library to implement multi_asic behaviour. Added relevant unit tests to ensure functionality.

How to verify it

Run unit tests, or the commands as listed above.

[vmittal-msft]

@bktsim-arista Can you please upate https://github.com/sonic-net/sonic-buildimage/issues/15148 on which PR is handling which command for easy tracking ?

[abdosi]

@alpeshspatel : please help review