[object_store] API to calculate memory consumption of `ObjectMeta`

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
In InfluxDB IOx we cache `ObjectMeta` (among other things) and thus calculate the memory used in order to limit the amount of memory used for caching

**Describe the solution you'd like**

In order to have accurate memory usage accounting, it would be nice to have an API like
```rust
impl ObjectMeta {
/// return the total bytes of memory used by this object, including heap space
pub fn memory_bytes(&self) -> usize {
...
}
}
```

**Describe alternatives you've considered**
We could cache the raw data directly as `Strings` and recreate `ObjectMeta` when needed

**Additional context**
See context in https://github.com/influxdata/influxdb_iox/pull/8577/files#r1316378812

Start by locating the Rust definition of ObjectMeta and reviewing how its stored data contributes to memory use. Add the requested memory_bytes API, with completion defined as reporting the total bytes used by an ObjectMeta instance, including heap space.

Written by the indexing model from the issue text.