[MCP] Add support for resources (#5239)

This PR adds support for [MCP
resources](https://modelcontextprotocol.io/specification/2025-06-18/server/resources)
by adding three new tools for the model:
1. `list_resources`
2. `list_resource_templates`
3. `read_resource`

These 3 tools correspond to the [three primary MCP resource protocol
messages](https://modelcontextprotocol.io/specification/2025-06-18/server/resources#protocol-messages).

Example of listing and reading a GitHub resource tempalte
<img width="2984" height="804" alt="CleanShot 2025-10-15 at 17 31 10"
src="https://github.com/user-attachments/assets/89b7f215-2e2a-41c5-90dd-b932ac84a585"
/>

`/mcp` with Figma configured
<img width="2984" height="442" alt="CleanShot 2025-10-15 at 18 29 35"
src="https://github.com/user-attachments/assets/a7578080-2ed2-4c59-b9b4-d8461f90d8ee"
/>

Fixes #4956

docs/config.md

@@ -377,6 +377,10 @@ experimental_use_rmcp_client = true
 url = "https://mcp.linear.app/mcp"
 # Optional environment variable containing a bearer token to use for auth
 bearer_token_env_var = "<token>"
+# Optional map of headers with hard-coded values.
+http_headers = { "HEADER_NAME" = "HEADER_VALUE" }
+# Optional map of headers whose values will be replaced with the environment variable.
+env_http_headers = { "HEADER_NAME" = "ENV_VAR" }
 ```
 
 For oauth login, you must enable `experimental_use_rmcp_client = true` and then run `codex mcp login server_name`