T6350: CGNAT add op-mode to show allocation (backport #3466)

[mergify]

Change Summary

Add op-mode command show nat cgnat allocation to get CGNAT allocations (internal address, external address, port-range)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://vyos.dev/T6350

Related PR(s)

Component(s) name

cgnat

Proposed changes

How to test

set nat cgnat pool external ext01 external-port-range '1024-65535'
set nat cgnat pool external ext01 per-user-limit port '2000'
set nat cgnat pool external ext01 range 192.168.122.222/32
set nat cgnat pool internal int01 range '100.64.0.0/28'
set nat cgnat rule 10 source pool 'int01'
set nat cgnat rule 10 translation pool 'ext01'

Show allocation:

vyos@r4:~$ show nat cgnat allocation 
Internal IP    External IP      Port range
-------------  ---------------  ------------
100.64.0.0     192.168.122.222  1024-3023
100.64.0.1     192.168.122.222  3024-5023
100.64.0.2     192.168.122.222  5024-7023
100.64.0.3     192.168.122.222  7024-9023
100.64.0.4     192.168.122.222  9024-11023
100.64.0.5     192.168.122.222  11024-13023
100.64.0.6     192.168.122.222  13024-15023
100.64.0.7     192.168.122.222  15024-17023
100.64.0.8     192.168.122.222  17024-19023
100.64.0.9     192.168.122.222  19024-21023
100.64.0.10    192.168.122.222  21024-23023
100.64.0.11    192.168.122.222  23024-25023
100.64.0.12    192.168.122.222  25024-27023
100.64.0.13    192.168.122.222  27024-29023
100.64.0.14    192.168.122.222  29024-31023
100.64.0.15    192.168.122.222  31024-33023
vyos@r4:~$ 

Smoketest result

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

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This is an automatic backport of pull request #3466 done by [Mergify](https://mergify.com).

[sever-sever]

We do not have CGNAT for 1.4