Improve MongoDB connection pooling and clustering

## Description
Enhance MongoDB connection management with advanced pooling and cluster support.

## Requirements
- [ ] Advanced connection pooling configuration
- [ ] Replica set support
- [ ] Sharded cluster support
- [ ] Read preference configuration
- [ ] Connection monitoring and health checks
- [ ] Write comprehensive tests

## Acceptance Criteria
- Robust connection pooling
- Full replica set support
- Sharded cluster compatibility
- Advanced read preferences

## Files to modify
- `beatrix_core/src/mongo.rs`
- `beatrix_core/src/mongo/connection.rs` (new file)

## Example Usage
```rust
let db = MongoDatabase::new("mongodb://host1,host2,host3/mydb?replicaSet=rs0")
.read_preference(ReadPreference::SecondaryPreferred)
.max_pool_size(50)
.connect().await?;
```