Load DCCL codec plugin shared libraries from AppConfig and GOBY_LOAD_SHARED_LIBRARY

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External DCCL codec plugins (libdccl_arithmetic.so, libdccl_native_protobuf.so) had no general loading path: only a handful of apps (pAcommsHandler, goby_bridge, goby_liaison, goby_logger) had a bespoke load_shared_library option, none of which registered codecs with the middleware DCCL codec used by the intervehicle layer, and gobyd had no option at all.

This adds a single loading mechanism at the goby::middleware::Application level, so it works for every Goby app including gobyd.

# config
app { load_shared_library: "libdccl_arithmetic.so" }

# or environment (':', ';' or ',' separated)
export GOBY_LOAD_SHARED_LIBRARY=libdccl_arithmetic.so:libdccl_native_protobuf.so

Changes

• AppConfig: new repeated string load_shared_library = 60 (ADVANCED). Each entry may itself contain several libraries separated by :, ; or ,, matching the existing ToolSharedLibraryLoader convention.
• DCCLSerializerParserHelperBase::load_library(): the std::string wrapper around dccl::Codec::load_library() is replaced by an implementation that dlopens with RTLD_LAZY | RTLD_NODELETE (consistent with the rest of Goby, and avoiding dccl's dlclose-on-destruct which can leave dangling Protobuf descriptor memory) and throws goby::Exception on failure. Handles are tracked and de-duplicated, and re-applied whenever the codec is created or replaced (codec() / set_codec()). A void* overload is added for already-opened handles.
• Application: new load_shared_libraries() invoked from the constructor after logger setup and before any DCCL use; reads GOBY_LOAD_SHARED_LIBRARY first, then config entries.
• ToolSharedLibraryLoader: libraries opened by goby zeromq publish/subscribe (-l / GOBY_TOOL_LOAD_SHARED_LIBRARY) are now also registered with the DCCL codec, so plugin codecs they contain are usable.
• Test src/test/middleware/dccl_load_library: a message using the external dccl.native_protobuf codec throws before loading and round-trips after; repeat loads are a no-op; a missing library throws.
• Docs: "Loading shared libraries (DCCL plugins)" section in doc230_application.md.

Notes for review

• GOBY_LOAD_SHARED_LIBRARY is read explicitly rather than via (goby.field).cfg.env, since env bindings are only generated for top-level config fields and AppConfig is nested under app.
• Existing per-app load_shared_library options are left untouched; this is additive.

• Fixes Need cleaner way to load external shared libraries containing DCCL plugins into apps and gobyd #266