Support clang format file with multiple languages

Right now we have a clang format file with multiple configurations for different languages.
With a call to `clang-format` this is working but not with `ament_clang_format`.

Example:

```
Language: Cpp
BasedOnStyle: Google
---
Language: Proto
BasedOnStyle: Google
```

Error:

```
Traceback (most recent call last):
File "/opt/ros/galactic/bin/ament_clang_format", line 11, in
load_entry_point('ament-clang-format==0.10.6', 'console_scripts', 'ament_clang_format')()
File "/opt/ros/galactic/lib/python3.8/site-packages/ament_clang_format/main.py", line 101, in main
data = yaml.safe_load(content)
File "/usr/lib/python3/dist-packages/yaml/__init__.py", line 162, in safe_load
return load(stream, SafeLoader)
File "/usr/lib/python3/dist-packages/yaml/__init__.py", line 114, in load
return loader.get_single_data()
File "/usr/lib/python3/dist-packages/yaml/constructor.py", line 49, in get_single_data
node = self.get_single_node()
File "/usr/lib/python3/dist-packages/yaml/composer.py", line 41, in get_single_node
raise ComposerError("expected a single document in the stream",
yaml.composer.ComposerError: expected a single document in the stream
in "", line 1, column 1:
Language: Cpp
^
but found another document
in "", line 55, column 1:
---
^

```