[backport camel-4.18.x] CAMEL-23547: camel-core - Properties component should eager start custom functions resolver

[davsclaus]

Backport of d1afebe onto camel-4.18.x

Cherry-pick of commit d1afebe onto camel-4.18.x.

Original commit: d1afebe - CAMEL-23547: camel-core - Properties component should eager start custom functions resolver. Same for jbang that has a custom resolver.
Target branch: camel-4.18.x

Changes

• DefaultPropertiesFunctionResolver: added doBuild() lifecycle method that calls ServiceHelper.buildService() on the registered functions, so they are properly built before use.
• KameletMain: the DependencyDownloaderPropertiesFunctionResolver is now eagerly started (via resolver.start()) before being set on the PropertiesComponent, ensuring the custom functions resolver is fully initialised before properties resolution begins. Applied to both the normal and export code paths.

Conflict resolution notes

The backport required minor adaptation: the 3-argument constructor added on main does not exist on camel-4.18.x, so the existing 2-argument constructor is used instead. The function registration in the constructor (which was moved to doBuild() on main) is left as-is on 4.18.x to minimise the delta.

Claude Code on behalf of Claus Ibsen