[MCP] Add support for streamable http servers with codex mcp add and replace bearer token handling

[gpeal]

1. You can now add streamable http servers via the CLI
2. As part of this, I'm also changing the existing bearer_token plain text config field with ane env var

mcp add github --url https://api.githubcopilot.com/mcp/ --bearer-token-env-var=GITHUB_PAT

[chatgpt-codex-connector]

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[bolinfest]

This doesn't look right to me. Chat suggested:

#[derive(Debug, clap::Args)]
pub struct AddMcpTransportArgs {
    #[command(flatten)]
    #[group(required = true, multiple = false, name = "transport")]
    pub stdio: Option<AddMcpStdioArgs>,

    #[command(flatten)]
    #[group(name = "transport")]
    pub streamable_http: Option<AddMcpStreamableHttpArgs>,
}

[gpeal]

Clap is a bit finicky to work with but I wrote unit tests to verify the failure and this change breaks a couple of them.