Add StreamableHTTPServerTransport documentation to README

[Copilot]

This PR adds comprehensive documentation for the existing HTTP transport functionality that was previously undocumented.

Changes Made

Documentation Updates

• Added HTTP Transport section to README explaining how to use StreamableHTTPServerTransport
• Documented command line options - added documentation for the -port flag to start HTTP mode
• Added code examples showing how to connect programmatically using StreamableHTTPClientTransport
• Updated development scripts documentation to include npm run dev:http
• Added environment variable notes explaining that LIGHTDASH_API_KEY and LIGHTDASH_API_URL must be set in the server environment for HTTP mode

Example Code

• Created examples/list_spaces_http.ts - a complete working example demonstrating HTTP client usage
• Shows proper imports and connection setup for StreamableHTTPClientTransport

Before/After

Before: README only documented stdio transport mode, with no mention of HTTP capabilities

After: Clear documentation for both transport modes:

# Stdio mode (default)
npx lightdash-mcp-server

# HTTP mode  
npx lightdash-mcp-server -port 8080

The server already supported both transports, but users had no way to discover or properly use the HTTP functionality.

Fixes #18.

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[syucream]

@copilot describe how to connect popular MCP clients like Claude Desktop to http mcp remote server, like the stdio case. I guess the clients now typically accept url field instead of command, arg, env

[Copilot]

@copilot describe how to connect popular MCP clients like Claude Desktop to http mcp remote server, like the stdio case. I guess the clients now typically accept url field instead of command, `arg...

Added Claude Desktop HTTP configuration documentation showing how to use the url field instead of command/args for connecting to the HTTP MCP server. The configuration is now documented alongside the programmatic example. Commit: ab59af0