[Store] Add metadata filtering support to InMemoryStore and CacheStore

[camilleislasse]

Q	A
Bug fix?	no
New feature?	yes
Docs?	yes
License	MIT

This PR adds metadata filtering support to local stores (InMemoryStore and CacheStore).

Changes

• Add optional filter parameter to query() method accepting a callable
• Filter documents by metadata before distance calculation
• Add comprehensive test coverage for filtering scenarios
• Add documentation for local stores

Usage

$results = $store->query($vector, [
     'filter' => fn(VectorDocument $doc) => $doc->metadata['size'] === 'S',
     'maxItems' => 10,
 ]);

[chr-hertel]

Thank you @camilleislasse.