Support custom auth schemes for MCP servers

[cecchi]

This PR adds an auth param to the MCPServerHTTP class, which is passed through to the underlying mcp library.

Livekit supports providing a list of static headers to the MCP client which is useful for basic authentication schemes, but are often insufficient. In our case, our authentication tokens are short-lived and need to be refreshed periodically. Having a more flexible way to configure authentication makes this possible.

session = agents.AgentSession(
    mcp_servers=[
        mcp.MCPServerHTTP(
            url="https://foo.bar/mcp/",
            auth=RefreshingAccessTokenAuth(),
        )
    ],
)

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