[Proposal / RFC] Align Mixin instance_init signature with Invocable instance_init signature

[eob]

Context

We've been discussing ways to simplify the Config handling requirements across mixins and packages.

One complexity identified is that configs of different types were being passed around automatically at different times.

This PR

This PR removes all arguments from the Mixin's instance_init method, bringing it into alignment with PackageService.

This simplifies the contract of a Mixin's state without impacting possibility: the state would not be provided exclusively upon instantiation, by the PackageService author. 100% of the state it needs must be provided at that time.

Mixins affected

This only affects the Telegram mixin, but it turns out that that Mixin already had -- via its __init__ method -- all of the information that it just lost by simplifying the instance_init method.

Implied "Best Practice" Mixin Configuration Pattern

This results in a situation where we do not need to commingle MixinConfig and PackageConfig.

The following manner of configuration can now keep the two configuration layers completely separate, ducking around multi-inheritance issues with Pydantic:

            TelegramTransport(
                client=self.client,
                config=TelegramTransportConfig(bot_token=self.config.bot_token),
            )

[dkolas]

Thanks! Looks good to me

[dkolas]

🚢