Have Oauth client library take settings overrides into account

Problem/Motivation

See #3167602-5: Document how to set up Oauth authentication.

GuzzleHttp\Exception\RequestException: cURL error 60: SSL certificate problem: unable to get local issuer certificate (see https://curl.haxx.se/libcurl/c/libcurl-errors.html) in GuzzleHttp\Handler\CurlFactory::createRejection() (line 201 of /project/vendor/guzzlehttp/guzzle/src/Handler/CurlFactory.php).
GuzzleHttp\Handler\CurlFactory::finishError(Object, Object, Object) (Line: 105)
GuzzleHttp\Handler\CurlFactory::finish(Object, Object, Object) (Line: 43)
GuzzleHttp\Handler\CurlHandler->__invoke(Object, Array) (Line: 28)
GuzzleHttp\Handler\Proxy::GuzzleHttp\Handler\{closure}(Object, Array) (Line: 51)
GuzzleHttp\Handler\Proxy::GuzzleHttp\Handler\{closure}(Object, Array) (Line: 66)
GuzzleHttp\PrepareBodyMiddleware->__invoke(Object, Array) (Line: 29)
GuzzleHttp\Middleware::GuzzleHttp\{closure}(Object, Array) (Line: 70)
GuzzleHttp\RedirectMiddleware->__invoke(Object, Array) (Line: 59)
GuzzleHttp\Middleware::GuzzleHttp\{closure}(Object, Array) (Line: 71)
GuzzleHttp\HandlerStack->__invoke(Object, Array) (Line: 351)
GuzzleHttp\Client->transfer(Object, Array) (Line: 112)
GuzzleHttp\Client->sendAsync(Object, Array) (Line: 129)
GuzzleHttp\Client->send(Object) (Line: 608)
League\OAuth2\Client\Provider\AbstractProvider->getResponse(Object) (Line: 621)
League\OAuth2\Client\Provider\AbstractProvider->getParsedResponse(Object) (Line: 537)
League\OAuth2\Client\Provider\AbstractProvider->getAccessToken(Object, Array) (Line: 307)
Drupal\entity_share_client\Plugin\ClientAuthorization\Oauth->initalizeToken(Object, Array) (Line: 251)
Drupal\entity_share_client\Plugin\ClientAuthorization\Oauth->submitConfigurationForm(Array, Object) (Line: 110)
Drupal\entity_share_client\Form\RemoteForm->submitForm(Array, Object)
call_user_func_array(Array, Array) (Line: 114)
Drupal\Core\Form\FormSubmitter->executeSubmitHandlers(Array, Object) (Line: 52)
Drupal\Core\Form\FormSubmitter->doSubmitForm(Array, Object) (Line: 593)
Drupal\Core\Form\FormBuilder->processForm('remote_edit_form', Array, Object) (Line: 321)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 91)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 573)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 151)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 68)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 67)
Drupal\simple_oauth\HttpMiddleware\BasicAuthSwap->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 52)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 708)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)
require('/project/app/index.php') (Line: 4)
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Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes