feat: Add support for mcp tools with rig agents + add anthropic prompt caching

[edisontim]

Allows developers to use mcp tools and servers with rig.

Example usage:

        use mcp_client_rs::client::ClientBuilder;

        let fetch_client = ClientBuilder::new("uvx")
            .arg("mcp-server-fetch")
            .spawn_and_initialize()
            .await?;
        let fetch_tools = fetch_client.list_tools().await?.tools;

        let fetch_client = Arc::new(fetch_client);
        
        for tool in fetch_tools {
            agent = agent.mcp_tool(tool, fetch_client.clone());
        }

[edisontim]

@cvauclair could you have a look at this please?