[Bug] iceberg-rust does not respect compression settings for metadata & avro

### Apache Iceberg Rust version

0.6

### Describe the bug

I'm using the Nessie catalog with the following settings:
```
- nessie.catalog.iceberg-config-defaults.write.avro.compression-level=3
- nessie.catalog.iceberg-config-defaults.write.avro.compression-codec=zstd
- nessie.catalog.iceberg-config-defaults.write.metadata.compression-codec=gzip
```
These settings are passed to the clients, but `iceberg-rust` does not have code that respects them.
As a result, it keeps producing uncompressed metadata and Avro files.

### To Reproduce

_No response_

### Expected behavior

_No response_

### Willingness to contribute

None

Start by tracing how the Nessie configuration keys write.avro.compression-level, write.avro.compression-codec, and write.metadata.compression-codec reach the metadata and Avro writers. Confirm the affected output currently lacks compression, then verify that the configured codecs and level are respected.

Written by the indexing model from the issue text.