Support Prompt protocol

[adhikasp]

Implement the prompt as per described in protocol https://modelcontextprotocol.io/docs/concepts/prompts

Usage

$ llm --list-prompts

<return list of prompts from all active MCP servers>

mcp server	name	description	arguments
mcp-youtube	summarize	summarize a youtube url in a concise but informative manner	youtube_url

Then we can use it with p keyword

$ llm p summarize https://www.youtube.com/watch?v=Fz1oMAMisgE

Trying to call undefined prompt or with not enough arguments should throw an error

If there is duplicated prompt name across mcp servers, throw ambiguous error