v6: add CollectionHandle .size() shortcut

[bevzzz]

Similarly to how Python's "collection" object has __len__ override, CollectionHandle will have .size() to retrieve collection size idiomatically.

var things = client.collections.use("Things");
System.out.printf("Collection Things has %d elements", things.size());

The -async version of this method is non-blocking.

resolves #423

[orca-security-eu]

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