feat: create libretime_shared package (#1349)

* feat: create libretime_shared package

- We don't use pydantic.BaseSettings because of some
  incompatble loading behavior.

* fix: whitelist pydantic in pylintrc

* docs: update to new BaseConfig behavior

* docs: change confusing config filepath

shared/README.md

@@ -0,0 +1,77 @@
+# Shared
+
+The `libretime_shared` package contains reusable functions and classes for the Libretime project.
+
+## Usage
+
+This library assumes that:
+
+- You will use [`Click`](https://github.com/pallets/click) to build a CLI for your app.
+- You will use [`Loguru`](https://github.com/delgan/loguru) to log messages from your app.
+- You will use [`Pydantic`](https://github.com/samuelcolvin/pydantic/) to validate objects in your app.
+
+### Configuration
+
+First define a schema for your configuration in order to validate it. A schema is a class that inherit from `pydantic.BaseModel`. Some existing schemas can be reused such as `libretime_shared.config.RabbitMQ` or `libretime_shared.config.Database`.
+
+Load your configuration using a subclass of `libretime_shared.config.BaseConfig`.
+
+```py
+from pydantic import BaseModel
+
+from libretime_shared.config import RabbitMQ, BaseConfig
+
+class Analyzer(BaseModel):
+    bpm_enabled: bool = False
+    bpm_track_max_length: int
+
+class Config(BaseConfig):
+    rabbitmq: RabbitMQ
+    analyzer: Analyzer
+
+config = Config(filepath="/etc/libretime/config.yml")
+```
+
+### App
+
+Create an app class that inherit from `libretime_shared.app.AbstractApp`.
+
+```py
+from libretime_shared.app import AbstractApp
+
+class LiquidsoapApp(AbstractApp):
+    name = "liquidsoap"
+
+    def __init__(self, some_arg, **kwargs):
+        super().__init__(**kwargs)
+        self.some_arg = some_arg
+
+    def run(self):
+        ...
+
+
+app = LiquidsoapApp(**kwargs)
+app.run()
+```
+
+### CLI
+
+Decorate your CLI commands with the shared decorators to add extra flags.
+
+```py
+import click
+from libretime_shared.cli import cli_logging_options, cli_config_options
+
+from .app import App
+
+@click.group()
+def cli():
+    pass
+
+@cli.command()
+@cli_config_options
+@cli_logging_options
+def run(**kwargs):
+    app = App(**kwargs)
+    return app.run()
+```