[Java SDK] Add comprehensive offset management for consumer groups

**Title:** `[Java SDK] Add comprehensive offset management for consumer groups`

**Description:**
Current SDK has limited offset management capabilities, making it difficult to implement proper checkpoint recovery in stream processors.

**Current State:**
- Basic auto-commit flag
- No manual offset control
- No seek capability

**Proposed Solution:**
```java
// Manual offset management
client.consumers().commitOffset(streamId, topicId, partition, offset);
client.consumers().seekToOffset(streamId, topicId, partition, offset);
client.consumers().seekToTimestamp(streamId, topicId, timestamp);

// Get current positions
Map positions = client.consumers().currentPositions(streamId, topicId);
```