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/Cargo.toml

@@ -0,0 +1,39 @@
+[package]
+edition.workspace = true
+license.workspace = true
+name = "codex-core-plugins"
+version.workspace = true
+
+[lib]
+doctest = false
+name = "codex_core_plugins"
+path = "src/lib.rs"
+
+[lints]
+workspace = true
+
+[dependencies]
+codex-app-server-protocol = { workspace = true }
+codex-config = { workspace = true }
+codex-core-skills = { workspace = true }
+codex-exec-server = { workspace = true }
+codex-git-utils = { workspace = true }
+codex-login = { workspace = true }
+codex-plugin = { workspace = true }
+codex-protocol = { workspace = true }
+codex-utils-absolute-path = { workspace = true }
+codex-utils-plugins = { workspace = true }
+dirs = { workspace = true }
+reqwest = { workspace = true }
+serde = { workspace = true, features = ["derive"] }
+serde_json = { workspace = true }
+tempfile = { workspace = true }
+thiserror = { workspace = true }
+tokio = { workspace = true, features = ["fs"] }
+toml = { workspace = true }
+tracing = { workspace = true }
+url = { workspace = true }
+
+[dev-dependencies]
+pretty_assertions = { workspace = true }
+tempfile = { workspace = true }