T7766: FRR: Support multiple labels in BGP-LU

[hedrok]

FRR: Support multiple labels in BGP-LU

This is my implementation of FRR support of multiple labels in BGP-LU. It is much easier to review as PR to FRR (see "Related PR(s)" below, I've rebased that branch to stable/10.2 with minimal conflict resolution.

Tested by:

• Manually on master of FRR
• Manually on 10.2 FRR (the patch in this PR)
• All FRR topotest passed for master

For testing ExaBGP was used, see topotest FRRouting/frr@40a900b for configuration.

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/T7766

Related PR(s)

FRR PR: FRRouting/frr#19961

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[github-actions]

👍
No issues in PR Title / Commit Title

[Copilot]

Pull Request Overview

This PR adds support for multiple labels in BGP-LU (BGP Labeled Unicast) as a patch file for FRRouting (FRR) package builds. The patch extends BGP-LU to handle label stacks of up to 10 labels instead of the previous single label limitation.

Key changes include:

• Increased BGP_MAX_LABELS from 2 to 10 to accommodate full label stacks in BGP-LU
• Refactored label handling functions to support multiple labels throughout the codebase
• Updated display and debugging output to show complete label stacks

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[ritika0313]

Review comments have been added to FRR repo PR at:
FRRouting/frr#19961