Add interface for etag caching

[sspat]

If request handler implements interface, is serving a GET request and has a If-None-Match header, the interface method eTag should be called before calling the request handler main method.

If the returned etag matches the etag from the request header "If-None-Match" - return 304 with empty body, if not - execute the request handler main method as usual.

Also, if request handler implements interface and is serving a GET request, the interface methods revalidate, cacheable, maxAge should be called before returning the response, setting the Cache-Control response header value based on the methods return values.

https://web.dev/http-cache/

Update tests, docs

<?php

declare(strict_types=1);

namespace OnMoon\OpenApiServerBundle\Interfaces;

use OnMoon\OpenApiServerBundle\Interfaces\Dto;

interface HTTPCached
{
    public const CACHE_NO_STORE = 'no-store';
    public const CACHE_NO_CACHE = 'no-cache';

    public const CACHEABLE_PUBLIC = 'private';
    public const CACHEABLE_PRIVATE = 'public';

    public function eTag(): string;

    /** @psalm-return HTTPCached::CACHE_NO_*|null */
    public function revalidate(): ?string;

    /** @psalm-return HTTPCached::CACHEABLE_* */
    public function cacheable(): string;

    public function maxAge(): int;
}