v6: Refactor REST endpoints and JSON de-/serialization

[bevzzz]

Closes #373

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tl;dr

• io.weaviate.client6.v1.collection namespace is replaced by io.weaviate.client.v1.api.
• io.weaviate.client6.internal namespace is replaced by io.weaviate.client.v1.internal.
• Check JSON.java and JSONTest.java for custom de-/serialization adapters.
• Integration test code adapted for some minor API changes, but largely unchanged.

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• Refactored HTTP-based endpoints to encapsulate request data in separate records and logic for building requests in Endpoint. This cleaned up WeaviateDataClient and WeaviateCollectionConfigClient a lot, letting these classes focus on the presenting the public API.
• Created type adapters for classes that require custom serialization (CustomTypeAdapterFactory.INSTANCE):
  1. WeaviateObject: references (beacons) are merged with properties
  2. WeaviateCollection: references (cross-reference definitions) are merged with properties
  3. VectorIndex: encode vector index kind (hnsw/flat/dynamic) separately and let the concrete configuration be handled by default Gson adapter
  4. Vectorizer: similarly to vector index, encode vectorizer kind (module) and let the concrete configuration of each module be handled by default Gson adapter
• Use a single static Gson instance under JSON.java -- it is thread-safe and keeps all configuration in one place.
• Added Async counterparts for each client:
  • WeaviateClient -> WeaviateDataClient
  • WeaviateClientAsync -> WeaviateDataClientAsync

The .async() idiom for creating an async client from v5 is preserved:

// Initialize directly
try (var asyncClient = new WeaviateClientAsync(config)) { ... }

// Or create from an instance of a sync client
var syncClient = new WeaviateClient(config);
try (var asyncClient = syncClient.async()) { ... }

Async client returns CompletableFuture of whatever the sync client returns (T).
This will let users manage async calls in a familiar manner, re-using any existing "machinery" for working with completable futures they may already have.

• Declare 3 generic parameters in WeaviateObject: P - properties, R - references, M - metadata. This allows us to re-use WeaviateObject for writing and reading objects. It might seem convoluted, but it provides a interop between different requests, e.g.:

// Fetch object
var person_1 = persons.query.byId("person-1");

// Create object
var person_2 = persons.data.insert(Map.of("name", "John Doe"));

// Use both as references
things.insert(Map.of("title", "Thing-1"), 
  thing -> thing.references(Reference.objects("hasOwner", person_1, person_2)));

• Dropped ObjectReference(objects: List<WeaviateObject>) wrapper in favor of a simpler List<WeaviateObject> design. There is no additional information that ObjectReference can encode and it only adds layers.
• Minor changes to gRPC response unmarshaler in QueryRequest.rpc() to reflect the new logic
• Fixed missing default argument in Aggregation.Builder which prevented running aggregation queries without requesting any metrics.

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