Support plugins to customize functionality

[rsyring]

Resolving an environment variable is done through a Resolver class.

We could add support for loading other Resolver classes through installed Python packages and/or module paths. E.g.:

resolvers:
  # This is a project name.  The resolver module path is read from the project's meta data.
  - env-config-bitwarden
  # This is a module path so resolvers can be loaded without installing a package.
  - myproj.libs.env_config:CustomResolver
profile:
  foo:
    # URL format isn't required, since Resolvers identify values they can resolver.  Obviously,
    # for this to work, env-config-bitwarden would need a resolver that can handle "bitwarden://"
    # paths.
    bar: 'bitwarden://work/myproj/bar'
    # And the custom resolver handles these
    baz: 'myproj://baz'

My guess is we'd use pluggy and can use pytest as an inspiration for how to integrate plugins.