Make all resource endpoints Conditionable

[onlime]

For enhanced DX, I have added Conditionable trait to all resource endpoints.

Usage BEFORE:

$endpoint = new LettersEndpoint($accessToken)
    ->setOrganisationId($organisationId);

if ($provider->isUsingStaging()) {
    $endpoint->useStaging();
}

with this PR:

$endpoint = new LettersEndpoint($accessToken)
    ->setOrganisationId($organisationId)
    ->when(
        $provider->isUsingStaging(),
        fn (LettersEndpoint $endpoint) => $endpoint->useStaging()
    );

providing fluent sexiness.