Vendor API connection code from Docker SDK for Python (#398)

* Vendor parts of the Docker SDK for Python

This is a combination of the latest git version
(https://github.com/docker/docker-py/tree/a48a5a9647761406d66e8271f19fab7fa0c5f582)
and the version before Python 2.7 support was removed
(https://github.com/docker/docker-py/tree/650aad3a5fb84059b392ad450f760ed08143ae3f),
including some modifications to work with Ansible module_utils's
system (i.e. third-party imports are guarded, and errors are
reported during runtime through a new exception
MissingRequirementException).

* Create module_utils and plugin_utils for working with the vendored code.

The delete call cannot be called delete() since that method already exists from requests.

* Vendor more code from Docker SDK for Python.

* Adjust code from common module_utils.

* Add unit tests from Docker SDK for Python.

* Make test compile with Python 2.6, but skip them on Python 2.6.

* Skip test that requires a network server.

* Add changelog.

* Update changelogs/fragments/398-docker-api.yml

Co-authored-by: Brian Scholer <1260690+briantist@users.noreply.github.com>

* Minimum API version is 1.25.

Co-authored-by: Brian Scholer <1260690+briantist@users.noreply.github.com>

plugins/module_utils/_api/credentials/utils.py

@@ -0,0 +1,58 @@
+# -*- coding: utf-8 -*-
+# This code is part of the Ansible collection community.docker, but is an independent component.
+# This particular file, and this file only, is based on the Docker SDK for Python (https://github.com/docker/docker-py/)
+#
+# Copyright (c) 2016-2022 Docker, Inc.
+#
+# It is licensed under the Apache 2.0 license (see Apache-2.0.txt in this collection)
+
+from __future__ import (absolute_import, division, print_function)
+__metaclass__ = type
+
+import os
+import sys
+
+from ansible.module_utils.six import PY2
+
+if PY2:
+    from distutils.spawn import find_executable as which
+else:
+    from shutil import which
+
+
+def find_executable(executable, path=None):
+    """
+    As distutils.spawn.find_executable, but on Windows, look up
+    every extension declared in PATHEXT instead of just `.exe`
+    """
+    if sys.platform != 'win32':
+        if PY2:
+            return which(executable, path)
+        else:
+            return which(executable, path=path)
+
+    if path is None:
+        path = os.environ['PATH']
+
+    paths = path.split(os.pathsep)
+    extensions = os.environ.get('PATHEXT', '.exe').split(os.pathsep)
+    base, ext = os.path.splitext(executable)
+
+    if not os.path.isfile(executable):
+        for p in paths:
+            for ext in extensions:
+                f = os.path.join(p, base + ext)
+                if os.path.isfile(f):
+                    return f
+        return None
+    else:
+        return executable
+
+
+def create_environment_dict(overrides):
+    """
+    Create and return a copy of os.environ with the specified overrides
+    """
+    result = os.environ.copy()
+    result.update(overrides or {})
+    return result