T6594: Add missed pppd_compat module (backport #3832)

[mergify]

Change Summary

Add missed pppd_compat module, used for accel-ppp daemons to call external scripts
We have extended scripts in CLI, but they do not call because the module is not loaded.

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/T6594

Related PR(s)

Component(s) name

IPoE, SSTP, PPTP, L2TP-server

Proposed changes

How to test

set service ipoe-server authentication mode 'noauth'
set service ipoe-server client-ip-pool ONE range '100.64.0.1-100.64.0.10'
set service ipoe-server client-ipv6-pool POOL-v6 delegate 2001:db8:8003::/48 delegation-prefix '56'
set service ipoe-server client-ipv6-pool POOL-v6 prefix 2001:db8::/64
set service ipoe-server default-ipv6-pool 'POOL-v6'
set service ipoe-server default-pool 'ONE'
set service ipoe-server extended-scripts on-up '/config/scripts/accel-pppd/on-up-flush.sh'
set service ipoe-server gateway-address '100.64.0.14/28'
set service ipoe-server interface eth1 network 'shared'
set service ipoe-server name-server '192.168.122.14'

Before the fix, after a subscriber connection (ipoe) the script on-ip-flush.sh is not executed

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 #3832 done by [Mergify](https://mergify.com).

[github-actions]

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Commit title 'IPoE-server: add missed pppd_compat module. . (cherry picked from commit b92bc20)' does not match the required format!. Valid title example: T99999: make IPsec secure

[github-actions]

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No issues in unused-imports