[MCP] Add support for MCP Oauth credentials (#4517)

This PR adds oauth login support to streamable http servers when
`experimental_use_rmcp_client` is enabled.

This PR is large but represents the minimal amount of work required for
this to work. To keep this PR smaller, login can only be done with
`codex mcp login` and `codex mcp logout` but it doesn't appear in `/mcp`
or `codex mcp list` yet. Fingers crossed that this is the last large MCP
PR and that subsequent PRs can be smaller.

Under the hood, credentials are stored using platform credential
managers using the [keyring crate](https://crates.io/crates/keyring).
When the keyring isn't available, it falls back to storing credentials
in `CODEX_HOME/.credentials.json` which is consistent with how other
coding agents handle authentication.

I tested this on macOS, Windows, WSL (ubuntu), and Linux. I wasn't able
to test the dbus store on linux but did verify that the fallback works.

One quirk is that if you have credentials, during development, every
build will have its own ad-hoc binary so the keyring won't recognize the
reader as being the same as the write so it may ask for the user's
password. I may add an override to disable this or allow
users/enterprises to opt-out of the keyring storage if it causes issues.

<img width="5064" height="686" alt="CleanShot 2025-09-30 at 19 31 40"
src="https://github.com/user-attachments/assets/9573f9b4-07f1-4160-83b8-2920db287e2d"
/>
<img width="745" height="486" alt="image"
src="https://github.com/user-attachments/assets/9562649b-ea5f-4f22-ace2-d0cb438b143e"
/>

docs/config.md

@@ -369,6 +369,8 @@ url = "http://127.0.0.1:3845/mcp"
 bearer_token = "<token>"
 ```
 
+Refer to the MCP CLI commands for oauth login
+
 ### Other configuration options
 
 ```toml
@@ -404,6 +406,12 @@ codex mcp get docs --json
 
 # Remove a server
 codex mcp remove docs
+
+# Log in to a streamable HTTP server that supports oauth
+codex mcp login SERVER_NAME
+
+# Log out from a streamable HTTP server that supports oauth
+codex mcp logout SERVER_NAME
 ```
 
 ## shell_environment_policy