bugfix: presigned configured urls

[stobrien89]

Description of changes:
Fixes issue with configured endpoint urls when used during presigning.

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[yenfryherrerafeliz]

I think readability can be improved here by using if statements instead. For example:

$builtIns['SDK::Endpoint'] = null;
if (isset($args['endpoint'])) {
    $builtIns['SDK::Endpoint'] = $args['endpoint'];
} else if(isset($config['configured_endpoint_url'])) {
    $builtIns['SDK::Endpoint'] = $this->getEndpoint();
}

Or if our docs build process would permit us:

$builtIns['SDK::Endpoint'] = args['endpoint'] ?? $config['configured_endpoint_url'] ?? null;

[yenfryherrerafeliz]

Also, why should we not consider the endpoint resolved by the ClientResolver utility instead?

For example, the client resolver will ignore resolving an endpoint if it was already provided in the arguments, otherwise it will resolve the endpoint with the default implementation there.
So then the resolved config should contains the endpoint we need:

aws-sdk-php/src/AwsClient.php

Line 226 in b4d98bf

$this->endpoint = new Uri($config['endpoint']);

What do you think?

[stobrien89]

We're unable to do that for historical reasons. The legacy endpoint provider will add an endpoint to the args returned from the clientResolver if that value is not already set, meaning the value is always set and we need a way to determine whether or not a customer has resolved an endpoint from their environment.

[stobrien89]

Re: the readability, happy to change that to an if/else.