Add hook_csp_policy_alter for themes

Problem/Motivation

Provide an alter hook enabling themes to alter the CSP policy.

Proposed resolution

/**
 * Alters the CSP policy.
 *
 * This hook is only invoked for themes, modules should add an event subscriber listening to the CspEvents::POLICY_ALTER event.
 *
 * @param \Drupal\csp\Csp $policy
 *   The CSP policy.
 * @param \Symfony\Component\HttpFoundation\Response $response
 *   The response the policy is applied to.
 */
function hook_csp_policy_alter(Csp $policy, Response $response): void {}

Remaining tasks

Invoke the hook and document it in csp.api.php.