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/utils/proxy.py

@@ -0,0 +1,84 @@
+# -*- 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
+
+from .utils import format_environment
+
+
+class ProxyConfig(dict):
+    '''
+    Hold the client's proxy configuration
+    '''
+    @property
+    def http(self):
+        return self.get('http')
+
+    @property
+    def https(self):
+        return self.get('https')
+
+    @property
+    def ftp(self):
+        return self.get('ftp')
+
+    @property
+    def no_proxy(self):
+        return self.get('no_proxy')
+
+    @staticmethod
+    def from_dict(config):
+        '''
+        Instantiate a new ProxyConfig from a dictionary that represents a
+        client configuration, as described in `the documentation`_.
+
+        .. _the documentation:
+            https://docs.docker.com/network/proxy/#configure-the-docker-client
+        '''
+        return ProxyConfig(
+            http=config.get('httpProxy'),
+            https=config.get('httpsProxy'),
+            ftp=config.get('ftpProxy'),
+            no_proxy=config.get('noProxy'),
+        )
+
+    def get_environment(self):
+        '''
+        Return a dictionary representing the environment variables used to
+        set the proxy settings.
+        '''
+        env = {}
+        if self.http:
+            env['http_proxy'] = env['HTTP_PROXY'] = self.http
+        if self.https:
+            env['https_proxy'] = env['HTTPS_PROXY'] = self.https
+        if self.ftp:
+            env['ftp_proxy'] = env['FTP_PROXY'] = self.ftp
+        if self.no_proxy:
+            env['no_proxy'] = env['NO_PROXY'] = self.no_proxy
+        return env
+
+    def inject_proxy_environment(self, environment):
+        '''
+        Given a list of strings representing environment variables, prepend the
+        environment variables corresponding to the proxy settings.
+        '''
+        if not self:
+            return environment
+
+        proxy_env = format_environment(self.get_environment())
+        if not environment:
+            return proxy_env
+        # It is important to prepend our variables, because we want the
+        # variables defined in "environment" to take precedence.
+        return proxy_env + environment
+
+    def __str__(self):
+        return 'ProxyConfig(http={0}, https={1}, ftp={2}, no_proxy={3})'.format(
+            self.http, self.https, self.ftp, self.no_proxy)