feat[pool]: Expose Close method to facilitate removal of broken connections

Expose `Close` method to allow connections to be closed if they are in a bad state.

There are situations were the acquired connection is in a broken state but there is no explicitly way to either remove it from the pool or close it.

As a package user I want to have the choice to close and destroy a broken connection.

The `backgroundHealthCheck` does not check for connection's state therefore it does not remove broken connections.

The snippet below exemplifies how I would use the Close method. After checking the connection state using Ping I can close/destroy in case of an error.

```go
conn, err := pool.Acquire(ctx)
if err != nil {
return nil, err
}

if err := conn.Ping(ctx); err != nil {
conn.Close() <<<<< This connections is broken and should not be put back into the pool using conn.Release()
return nil, level.Error(logger).Log("msg", "failed to ping acquired connection from pool", "err", err)
}

return conn, nil
```

Please let me know if there is an alternative way of getting the same behaviour.