[feat](fe) Add ClusterGuard SPI interface for cluster-level policy enforcement

[morningman]

What problem does this PR solve?

Issue Number: N/A

Problem Summary:
Add a ClusterGuard SPI (Service Provider Interface) abstraction in fe-core to
decouple cluster-level policy enforcement (e.g., node limits, time validity checks)
from business logic. The open-source fe-core depends only on the interface, while
commercial implementations can be plugged in at runtime via Java ServiceLoader
without any compile-time dependency on fe-core.

Key changes:

• Add ClusterGuard interface with onStartup, checkTimeValidity, checkNodeLimit and getGuardInfo methods
• Add ClusterGuardFactory using Java ServiceLoader to discover implementations at runtime
• Add NoOpClusterGuard as the open-source no-op default implementation
• Add ClusterGuardException for guard-related error reporting
• Add ClusterGuardAction HTTP API at /api/cluster_guard/ for status query
• Integrate ClusterGuardFactory.getGuard() into DorisFE.java (startup) and SystemInfoService.java (addBackend/dropBackend)

Release note

None

Check List (For Author)

• Test: No need to test (internal SPI wiring, no behavior change in open-source build)
• Behavior changed: No
• Does this need documentation: No

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

[hello-stephen]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[morningman]

run buildall

[morningman]

run feut

[hello-stephen]

FE UT Coverage Report

Increment line coverage 33.01% (34/103) 🎉
Increment coverage report
Complete coverage report