Plugin mechanism for registering extra commands

[simonw]

Inspired by Datasette's: https://docs.datasette.io/en/stable/plugins.html

Refs:

• A model that picks the right sized model #32
• llm models command #33
• Add support for Google's PaLM 2 #20

Three initial plugin hooks:

• register_commands(cli) - to register extra commands
• Plugin hook: register_templates - return a list of llm.Template #52
• Plugin hook: register_models #53

[simonw]

Initial documentation: https://github.com/simonw/llm/blob/945c5026a0c0ef6808f81a804372b95631b6aace/docs/plugins.md

[simonw]

First plugin is here: https://github.com/simonw/llm-hello-world

[simonw]

I'm going to build the other plugin hooks in separate tickets.

• Plugin hook: register_models #53
• Plugin hook: register_templates - return a list of llm.Template #52