Add a cache: constructor argument to Client and CallbackHandler

[loevgaard]

Part of #10 (finding 7).

Why

Wiring a Valinor cache — the recommended production setup — meant building your own MapperBuilder/NormalizerBuilder and remembering to run them through Client::configureMapperBuilder() / registerNormalizerTransformers(). Forgetting that compiles fine and fails at runtime (dates don't map, superfluous keys throw). That's a footgun on the happy path.

What

• Client::__construct(..., ?Cache $cache = null) — applied to the SDK's default mapper and normalizer builders. Ignored for a builder passed explicitly (those are used as given, exactly as before), which the docblock spells out.
• CallbackHandler::__construct(string $privateKey, ?MapperBuilder $mapperBuilder = null, ?Cache $cache = null) — same idea for the callback side.
• Client::defaultMapperBuilder(?Cache) / defaultNormalizerBuilder(?Cache) are now public (the callback side uses the former; also handy for consumers who map raw arrays themselves).
• README "Production usage" now leads with the two-liner and explains when (and how) to pass a builder instead.

$cache = new FileSystemCache(__DIR__ . '/var/cache/valinor');
$client = new Client('YOUR_API_KEY', cache: $cache);
$handler = new CallbackHandler('YOUR_PRIVATE_KEY', cache: $cache);

Purely additive (new optional trailing parameters). Tests: the cache directory is populated through both mapping and normalizing on the client and through the handler; the default builders map the SDK's date formats and strip nulls with and without a cache. (QuickpayTestCase gains tempDir()/removeDir() helpers.)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.03%. Comparing base (fece712) to head (8e22262).

Additional details and impacted files

@@             Coverage Diff              @@
##                1.x      #16      +/-   ##
============================================
+ Coverage     99.02%   99.03%   +0.01%     
- Complexity      190      192       +2     
============================================
  Files            26       26              
  Lines           511      517       +6     
============================================
+ Hits            506      512       +6     
  Misses            5        5              

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