Change avro schema names for `ManifestList` and `Manifest` to the official ones

[zeodtr]

Currently, the avro schema names in the source code are as follows:

• ManifestList: "manifest_list"
• Manifest: "manifest"

(Search for schema_to_avro_schema function calls to see them)
But official names in the spec(https://iceberg.apache.org/spec/) are as follows:

• ManifestList: "manifest_file"
• Manifest: "manifest_entry"

This difference makes Iceberg Spark SQL (using spark-3.2.2-bin-hadoop3.2.tgz and iceberg-spark-runtime-3.2_2.12-1.4.3.jar) cannot read the manifest list files and manifest files that are created by iceberg-rust.
For example, Iceberg Spark SQL's error message for ManifestFile is as follows:
org.apache.iceberg.shaded.org.apache.avro.generic.GenericData$Record cannot be cast to org.apache.iceberg.ManifestFile

When I changed the avro schema names to the official ones, those errors were gone.

[JanKaul]

I would like to take this one. It was me who introduced the wrong names.

[liurenjie1024]

Thanks @zeodtr for reporting this! And thanks @JanKaul for taking it.