os-frr: add neighbor local-as no-prepend/replace-as options

[clarkchentw]

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Is your feature request related to a problem? Please describe.
Neighbor-level local-as was added in #5308, but FRR's no-prepend and replace-as modifiers are not exposed in the BGP neighbor UI.

Without those modifiers, FRR prepends the Local AS to received AS-Paths and still uses the real AS in outbound updates. That is often not the desired behavior for ASN migrations or provider peering such as AWS Direct Connect.

Describe the solution you'd like
Add an optional per-neighbor Local AS Options field in Routing -> BGP neighbor configuration, supporting:

• no-prepend
• no-prepend replace-as

Example FRR config:

router bgp 65000 neighbor 203.0.113.2 remote-as 64496 neighbor 203.0.113.2 local-as 65123 no-prepend replace-as

Describe alternatives you've considered
Manual vtysh configuration. That is not a proper solution because OPNsense regenerates FRR config and those changes may be lost.

Additional context
Follow-up to #5307 / #5308. FRR already supports these modifiers natively; this only exposes them in the plugin UI.