context UPDATE run-time plugin support (#2213)

* Adding context based extensions plugins support

This patch adds ability to load plugins directly from memory without
need to create shared library by using lyplg_add_plugin() API.

It also allows to associate plugin directly with context, so given
plugin will not affect all contexts, just given context

* Refactored based on PR comments
diff --git a/src/tree_schema.c b/src/tree_schema.c
index e8b9857..9d9d2b4 100644
--- a/src/tree_schema.c
+++ b/src/tree_schema.c
@@ -1362,7 +1362,7 @@
             LY_CHECK_RET(lysp_ext_instance_resolve_argument(PARSER_CTX(pctx), ext));
 
             /* find the extension record, if any */
-            ext->record = lyplg_ext_record_find(mod->name, mod->revision, ext->def->name);
+            ext->record = lyplg_ext_record_find(mod->ctx, mod->name, mod->revision, ext->def->name);
         }
     }