feat: add Airtel Money support, token caching, and checkAccountHolder

[lepresk]

Summary

• Add full Airtel Money API support: AirtelApi, AirtelCollectionApi, AirtelDisbursementApi
• Add AirtelConfig and AirtelTransaction models
• Add in-memory token caching (TokenCache) to avoid redundant OAuth requests
• Add checkAccountHolder() to CollectionApi and DisbursementApi
• 76 tests passing (15 new tests for Airtel + checkAccountHolder + caching)

What's new

Airtel Money

$collection = AirtelApi::collection('staging', AirtelConfig::collection(
    clientId: 'YOUR_CLIENT_ID',
    clientSecret: 'YOUR_CLIENT_SECRET',
));
$externalId = $collection->requestToPay('5000', '068511358', 'ORDER-001');

Check Account Holder (MTN)

$active = $momo->collection()->checkAccountHolder('242068511358'); // true/false

Token Caching

Access tokens are now cached per-instance for their TTL (minus 60s buffer), eliminating redundant auth round-trips.