Define appsignal.start() initialization method

This allows for a more ergonomic replacement to the previous:

```python
from __appsignal__ import appsignal
appsignal.start()
```

Into a simpler:

```python
import appsignal
appsignal.start()
```

But, more importantly, it allows users to keep the AppSignal
configuration file at the project root (where commands such as
`appsignal diagnose` expect to find it) and refer it from other
parts of their application (such as `project_name/wsgi.py` in a
Django project) without imposing the demand for the project root
to itself be a Python package.

src/appsignal/__init__.py

@@ -1,3 +1,8 @@
+from __future__ import annotations
+
+import os
+from runpy import run_path
+
 from .client import Client as Appsignal
 from .metrics import add_distribution_value, increment_counter, set_gauge
 from .tracing import (
@@ -37,9 +42,63 @@
     "increment_counter",
     "set_gauge",
     "add_distribution_value",
+    "start",
 ]
 
 
+# Load the AppSignal client from the app specific `__appsignal__.py` client
+# file. This loads the user config, rather than our default config.
+# If no client file is found it return `None`.
+# If there's a problem with the client file it will raise an
+# `InvalidClientFileError` with a message containing more details.
+def _client_from_config_file() -> Appsignal | None:
+    cwd = os.getcwd()
+    app_config_path = os.path.join(cwd, "__appsignal__.py")
+    if os.path.exists(app_config_path):
+        try:
+            client = run_path(app_config_path)["appsignal"]
+            if not isinstance(client, Appsignal):
+                raise InvalidClientFileError(
+                    "The `appsignal` variable in `__appsignal__.py` does not "
+                    "contain an AppSignal client. "
+                    "Please define the configuration file as described in "
+                    "our documentation: "
+                    "https://docs.appsignal.com/python/configuration.html"
+                )
+
+            return client
+        except KeyError as error:
+            raise InvalidClientFileError(
+                "No `appsignal` variable found in `__appsignal__.py`. "
+                "Please define the configuration file as described in "
+                "our documentation: "
+                "https://docs.appsignal.com/python/configuration.html"
+            ) from error
+
+    return None
+
+
+def _must_client_from_config_file() -> Appsignal:
+    client = _client_from_config_file()
+    if client is None:
+        raise InvalidClientFileError(
+            "No `__appsignal__.py` file found in the current directory. "
+            "Please define the configuration file as described in "
+            "our documentation: "
+            "https://docs.appsignal.com/python/configuration.html"
+        )
+
+    return client
+
+
+def start() -> None:
+    _must_client_from_config_file().start()
+
+
+class InvalidClientFileError(Exception):
+    pass
+
+
 # Try and load the appsignal-beta package. If it's present and imported, it
 # will print a message about switching to the `appsignal` package.
 try:

src/appsignal/cli/command.py

@@ -5,10 +5,10 @@
 from argparse import ArgumentParser, Namespace
 from dataclasses import dataclass
 
+from .. import _client_from_config_file
 from ..client import Client
 from ..config import Config, Options
 from ..push_api_key_validator import PushApiKeyValidator
-from .config import _client_from_config_file
 from .exit_error import ExitError