Discover additional flag definitions via ServiceLoader SPI

[esolitos]

What

Add a FlagDefinitions service-provider interface (single register()
method). Flags.<clinit> and PermanentFlags.<clinit> discover
implementations via java.util.ServiceLoader and invoke register()
once per JVM, after the static flag map is initialized. Existing flag
definitions are untouched.

Why

• Lets other bundles on the classpath contribute flag definitions without
  modifying this one.
• Keeps consumer-specific metadata (owners, expiry dates, rollout notes)
  out of this generic bundle.
• Purely additive: no new compile-time or runtime edge from this bundle
  to any other artifact, and no behavior change for flags currently
  defined here.

[Copilot]

Pull request overview

Adds a ServiceLoader-based extension point so external artifacts on the classpath can contribute additional flag definitions to the central Flags registry, with tests verifying discovery and interaction with Flags.Replacer.

Changes:

• Introduce FlagDefinitions SPI (single register() method) for out-of-bundle flag registration.
• Load and invoke FlagDefinitions providers during Flags class initialization (and trigger from PermanentFlags).
• Add test-only SPI implementation + ServiceLoader registration and regression tests for visibility and Flags.Replacer behavior.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
flags/src/main/java/com/yahoo/vespa/flags/FlagDefinitions.java	Adds the SPI contract for registering external flag definitions.
flags/src/main/java/com/yahoo/vespa/flags/Flags.java	Loads FlagDefinitions via ServiceLoader once and runs providers during static init.
flags/src/main/java/com/yahoo/vespa/flags/PermanentFlags.java	Calls into Flags to ensure SPI loading when PermanentFlags is initialized.
flags/src/test/resources/META-INF/services/com.yahoo.vespa.flags.FlagDefinitions	Registers the test SPI implementation on the test classpath.
flags/src/test/java/com/yahoo/vespa/flags/TestFlagDefinitions.java	Test-only FlagDefinitions provider that registers sample flags.
flags/src/test/java/com/yahoo/vespa/flags/FlagDefinitionsSpiTest.java	Verifies ServiceLoader discovery and SPI-registered flags are visible; checks no double-registration.
flags/src/test/java/com/yahoo/vespa/flags/FlagsReplacerSpiRegressionTest.java	Regression tests ensuring Flags.Replacer saves/restores SPI-registered flags correctly.

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[esolitos]

As discussed, let's find another way.