Add subtree registration system

[zeerekahmad]

Description

Adds file-based subtree (behavior tree XML) plugin support. Plugin packages can now ship reusable subtree .xml files that BT::PluginAwareFactory auto-discovers and registers on construction, so any tree loaded afterward can reference them "for free" via <SubTree ID="..."/> — mirroring how node plugins already work.

• PluginAwareFactory gains a second load step (after node registration) that discovers subtree XML through a new behaviortree_cpp_subtrees ament resource-index category and registers each via registerBehaviorTreeFromFile. A malformed or unresolvable subtree is logged and skipped rather than aborting construction.
• Ordering is intentional: all node types register before any subtree XML, because BT.CPP verifies XML at registration time and rejects subtrees referencing an unregistered node. <SubTree> references resolve lazily, so subtree-to-subtree order doesn't matter.
• New CMake API: a SUBTREES <files...> keyword on register_behaviortree_cpp_plugin() for subtrees co-shipped with a node plugin, plus a standalone register_behaviortree_cpp_subtrees(NAME <suffix> FILES <files...>) for node-less packages. Both install the XML and register a per-call-unique ament marker.
• Subtrees are treated as pure data — no change to BT::BehaviorTreePlugin — so no plugin-base vtable/ABI change and already-compiled plugin .sos stay compatible. ament_index_cpp is added as a PRIVATE dependency.

Migration Guide

None — the change is purely additive. Existing plugins and consumers build and behave unchanged; the new SUBTREES keyword and register_behaviortree_cpp_subtrees() function are optional.

Related issues

Closes #3

Completeness checklist

• Is there test coverage? If it was a bugfix, is there a regression test?
• Is user-facing API/behavior documented?
• Is core documentation updated?
• Are all checks green?
• Assign reviewers to the PR and post in #code_reviews

[emersonknapp]

one request: an addition to README.md about this feature