net/frr: Add per-neighbor local-as no-prepend/replace-as options

[clarkchentw]

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Describe the problem

Neighbor-level local-as was added in #5308, but FRR's no-prepend and replace-as modifiers are still missing. Without them, the configured Local AS is prepended to received AS-Paths, and the real AS remains in outbound updates. That is often not the desired behavior for ASN migrations or provider peering such as AWS Direct Connect.

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Describe the proposed solution

This pull request adds optional per-neighbor Local AS modifiers:

• Add localasoptions to the BGP neighbor model (no-prepend, no-prepend replace-as)
• Add an advanced Local AS Options dropdown next to Local AS
• Require Local AS when Local AS Options are set
• Render neighbor <address> local-as <asn> [no-prepend [replace-as]] in bgpd.conf when configured

The change is backward compatible: no extra keywords are emitted unless explicitly configured.

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Related issue

Closes #5687

[Monviech]

This will most likely make it in in some form or another at some point.

Currently Im working on cleaning up the BGP templates, so new features should be deferred until that work has been done.

The current features should work as expected before adding new ones.

So I'm putting this in the queue but it might take a while still.