Extract plugin loading and marketplace logic into codex-core-plugins (#18070)

Split plugin loading, marketplace, and related infrastructure out of
core into codex-core-plugins, while keeping the core-facing
configuration and orchestration flow in codex-core.

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core-plugins/BUILD.bazel

@@ -0,0 +1,15 @@
+load("//:defs.bzl", "codex_rust_crate")
+
+codex_rust_crate(
+    name = "core-plugins",
+    crate_name = "codex_core_plugins",
+    compile_data = glob(
+        include = ["**"],
+        exclude = [
+            "**/* *",
+            "BUILD.bazel",
+            "Cargo.toml",
+        ],
+        allow_empty = True,
+    ),
+)