T6219: Add support for container sysctl parameter (backport #3614)

[mergify]

I can't append to #3288, so I took the liberty to build upon it.
Aligned configuration with system sysctl and added completion with allowed sysctl values for container

Change Summary

Allow setting sysctl settings for containers

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/T6219

Related PR(s)

#3288

Component(s) name

container

Proposed changes

Adds support for configuring a container with the sysctl option:

container {
    name c1 {
        allow-host-networks
        image "busybox:stable"
        sysctl {
            parameter kernel.msgmax {
                value "8192"
            }
        }
    }
}

How to test

set container name c1 image busybox:stable
set container name c1 allow-host-networks
set container name c1 sysctl parameter kernel.msgmax value 8192

Smoketest result

test_basic (__main__.TestContainer.test_basic) ... ok
test_dual_stack_network (__main__.TestContainer.test_dual_stack_network) ... 
IP address "192.0.2.1" can not be used for a container, reserved for the
container engine!

ok
test_ipv4_network (__main__.TestContainer.test_ipv4_network) ... 
IP address "192.0.2.1" can not be used for a container, reserved for the
container engine!

ok
test_ipv6_network (__main__.TestContainer.test_ipv6_network) ... 
IP address "2001:db8::1" can not be used for a container, reserved for
the container engine!

ok
test_uid_gid (__main__.TestContainer.test_uid_gid) ... 
Cannot set "gid" without "uid" for container

ok

----------------------------------------------------------------------
Ran 5 tests in 146.650s

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 #3614 done by [Mergify](https://mergify.com).