os-frr: add support for per-neighbor local-as in BGP

[danohn]

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Describe the bug
The os-frr plugin does not expose a per-neighbor local-as option in the BGP neighbor configuration, even though FRR supports it natively.

This makes it impossible to configure a different presented local ASN for only one peer from the OPNsense UI. This is useful for migration, interoperability, and temporary transition scenarios.

Last known working version: not aware of any version where this was available in the OPNsense os-frr UI.

To Reproduce
Steps to reproduce the behavior:

1. Go to Routing -> BGP
2. Edit or create a BGP neighbor
3. Review the available neighbor options
4. Observe there is no option to configure neighbor-specific local-as

Expected behavior
There should be a per-neighbor option in the BGP neighbor configuration to set local-as.

For example, FRR supports configuration like:

router bgp 65000
 neighbor 203.0.113.2 remote-as 64496
 neighbor 203.0.113.2 local-as 65123

Screenshots
Not applicable.

Relevant log files
Not applicable.

This is a missing UI/configuration capability rather than a runtime log error.

Additional context
This can currently be worked around temporarily using vtysh, but that is not a proper solution because OPNsense regenerates FRR configuration and manual changes may be lost.

This appears to be a feature gap in the os-frr plugin rather than a limitation in FRR itself.

Environment
OPNsense [26.1.3] (amd64)