Add drand-client-php (PHP) to Client Libraries list

### New client library: **drand-client-php**

* **Repository:** https://github.com/dev404ai/drand-client-php
* **Package:** `dev404ai/drand-client-php` (Composer)
* **Language / Runtime:** PHP ≥ 8.2
* **License:** This project is dual-licensed under the MIT and Apache 2.0 licenses
* **Maintainer:** @dev404ai (dev404ai@gmail.com)

#### Key features
- Pure-PHP HTTP client (no native extensions)
- Verifies BLS12-381 signatures against chain-hash
- Automatic fail-over & racing across League-of-Entropy endpoints
- PSR-18 HTTP client / PSR-17 factory agnostic
- Optional PSR-6 / PSR-16 caching layer
- 100 % PHPUnit coverage, PHPStan level max

```php
use Drand\Client\Http\DrandHttpClient;

$drand = new DrandHttpClient(
'https://api.drand.sh',
// League-of-Entropy default chain hash
'8990e7a9aaed2ffed73dbd7092123d6f289930540d7651336225dc172e51b2ce'
);

$latest = $drand->latest();
printf("Round %d – %s\n", $latest->round, $latest->randomness);
```

We’d love to have this library listed under Developer → Client Libraries on docs.drand.love.
Happy to open a follow-up PR adding a row to developer/clients.md once approved.

Thanks for your time!

/cc @nikkolasg @alanshaw @anorth