feat: add label field to provider options

[typpo]

Right now when you want to change the label on a provider, you have to use this funky construction with a map:

providers:
  - openai:gpt-3.5-turbo-0613:
      id: openai-gpt-3.5-turbo-lowtemp
      config:
        temperature: 0

This change adds a label field, which allows you to use the regular list of objects

  - id: openai:gpt-3.5-turbo-0613
    label: openai-gpt-3.5-turbo-lowtemp
    config:
      temperature: 0

Eventually I'd like to get rid of support for the map, because it's unnecessary and inconsistent.

Related to #511