Update W-ECMP CLI for more options

[colintle]

Associated PR: https://github.com/sonic-net/sonic-buildimage/pull/19529 https://github.com/sonic-net/sonic-buildimage/pull/19530

What I did

I updated W-ECMP cli to include more than just num-multipaths. This PR includes cumulative and custom weight assignments

How I did it

I modified existing code to now include more subcommands of "config bgp device-global w-ecmp"

How to verify it

I wrote unit tests for this and tested it on virtual testbed

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)