Enhance `GetPoolStats()` Method to Include `Num_bytes_available` Field in `PoolStat`

**Description**

Currently, the "github.com/ceph/go-ceph/rados" Go library provides a convenient way to interact with Ceph, including retrieving pool statistics using the `GetPoolStats` method. However, the `Num_bytes_available` field is missing in the retrieved pool statistics.

In the current `PoolStat` struct, there is a `Num_bytes` field that represents the space used in bytes, but there is no corresponding field representing the available space (`Num_bytes_available`).

This feature request proposes the addition of support for the `Num_bytes_available` field in the `PoolStat` struct and the corresponding `GetPoolStats` method. Including the `Num_bytes_available` field is important for users who want to have comprehensive insights into the pool's status, including both used and available capacity.

**Proposed Changes**

- Modify the Ceph C library to include support for retrieving the `Num_bytes_available` field in pool statistics.
- Update the Go bindings in "github.com/ceph/go-ceph/rados" to expose this new functionality to Go code by extending the `PoolStat` struct to include a `Num_bytes_available` field.
- Ensure comprehensive testing to verify the correct retrieval of the `Num_bytes_available` field.
- Update the documentation to include information about how to use the new `Num_bytes_available` functionality.

**Use Case**

The addition of the `Num_bytes_available` field will benefit users who need to monitor and manage Ceph pools, especially when making decisions based on available capacity.

**Community Impact**

This enhancement will improve the completeness and usability of the "github.com/ceph/go-ceph" library and benefit the broader Ceph community.

**Additional Information**

Current `PoolStat` struct definition:
```go
type PoolStat struct {
// space used in bytes
Num_bytes uint64
// space used in KB
Num_kb uint64
// number of objects in the pool
Num_objects uint64
// number of clones of objects
Num_object_clones uint64
// num_objects * num_replicas
Num_object_copies uint64
Num_objects_missing_on_primary uint64
// number of objects found on no OSDs
Num_objects_unfound uint64
// number of objects replicated fewer times than they should be
// (but found on at least one OSD)
Num_objects_degraded uint64
Num_rd uint64
Num_rd_kb uint64
Num_wr uint64
Num_wr_kb uint64
}

Start with the rados PoolStat definition and GetPoolStats binding described in the issue, then trace how pool statistics are obtained from the Ceph C library. Add and verify the available-bytes field, and update the related documentation; completion requires tests demonstrating that the value is retrieved correctly.

Written by the indexing model from the issue text.