Provider registry

[brichet]

This PR replaces the AIProvider to a registry, to allow third party extensions to add providers.
Fixes #45

This registry is provided using a token, and providers can be added to it.
A provider must have the following interface:

export interface IAIProvider {
  /**
   * The name of the provider.
   */
  name: string;
  /**
   * The chat model class to use.
   */
  chatModel?: IType<BaseChatModel>;
  /**
   * The completer class to use.
   */
  completer?: IType<IBaseCompleter>;
  /**
   * the settings schema for the provider.
   */
  settingsSchema?: any;
  /**
   * The instructions to be displayed in the settings, as helper to use the provider.
   * A markdown renderer is used to render the instructions.
   */
  instructions?: string;
  /**
   * A function that extract the error message from the provider API error.
   * Default to `(error) => error.message`.
   */
  errorMessage?: (error: any) => string;
}

Most of the fields are optional because a provider can do only completion or chat, and can required no settings to work.

[brichet]

In follow up PRs (to reduce the size of this PR's changes) we could:

• reorganize the repository for a better readability, to have a default-providers directory. This directory could contain one directory per provider, each including the completer class, the generated settings and the instructions

  - default-providers
    - MistralAI
      - completer.ts
      - instructions.md
      - settings.json
    - OpenAI
      - ...
  

• add one plugin per 'default provider', to allow third party extensions to disable them

[jtpio]

Thanks!