Feat: Add pyaml-env constructor PyamlEnvConstructor

[dougppaz]

Simple way to add !ENV constructor to a pyyaml loader.

from yaml import Loader
from pyaml_env import  PyamlEnvConstructor

PyamlEnvConstructor.add_to_loader_class(loader_class=Loader)
# or
PyamlEnvConstructor.add_to_loader_class(
  loader_class=Loader,
  tag=custom_tag,
  sep=custom_sep,
  default_value=custom_default_value,
  raise_if_na=True
)

Motivation: a pyaml loader can have multiple custom constructors and many any ways to load a YAML content.

[mkaranasou]

Nice 👍, thanks for the addition! Besides the one comment for the relative imports, do you think it would be beneficial to add any tests for your change?

[olegvg]

Hi, @dougppaz @mkaranasou
It would be great to merge this PR despite the insufficiency of test coverage eventually. I've found this PR helpful with pydantic_yaml, which is becoming aligned with the 12-factor methodology.