Enable compatibility with new fastapi-slim and update examples

[azogue]

cc @sabuhish

closes #91

Maintenance update, with minor changes:

• Updated examples with lifespan usage (instead of deprecated .init_app() method)
• Better typing for decorators
• Increase required Python version to >=3.8 (syncing with fastAPI)
• Adjust dependencies to work with new fastapi-slim package variant, available since fastAPI v0.111.0

Here in fastapi-mqtt we only use from fastapi import FastAPI for testing and typing, so it is not a required dependency after all.

Moving it to the dev-dependencies group, with pytest and others, enables the usage of this library in projects using only the new fastapi-slim package, remaining compatible with the 'normal' fastapi

All changes

• 🎨 lint: pre-commit autoupdate
• ♻️ env: Adjust linter configuration in pyproject.toml, adding mypy strict settings
• 🎨 typing: Add missing detailed callable definitions
• 🎨 lint: Add missing typing
• 🎨 Validate MQTT protocol version in config
• 🐛 examples: Fix typos (closes Error found on the example page of the document #89)
• 🎨 Update all examples to use modern fastapi lifespan
• 🎨 Remove import of FastAPI object inside library, as it's only used for typing inside a deprecated method
• 📦️ env: Bump minor version and re-define deps and classifiers, moving fastapi to dev-dependencies, as it is used/required only for testing, and enables compatibility with fastapi-slim package, so projects using this library are not forced to install the full package. Same goes for uvicorn, which is an optional import to configure a logger