Missing support for custom attributes on primitive types

Avro allows attaching custom properties to any type, including primitives.

In avro-rs, primitive schemas like `Schema::Long`/`Schema::Int`/etc are modeled as bare enum variants and there is no field to store custom properties. `Schema::custom_attributes()` also returns None for primitives.

As a result, parsing an object-form primitive schema silently drops all extra properties, and serializing the parsed schema emits just "long", so the metadata cannot be inspected or round-tripped.

reproducer:
```rust
use apache_avro::Schema;
use apache_avro_test_helper::TestResult;
use serde_json::json;

#[test]
fn test1() -> TestResult {
let input = json!({
"type": "long",
"custom-prop": "value"
});

let schema = Schema::parse(&input)?;
assert!(matches!(schema, Schema::Long));

let serialized = serde_json::to_string(&schema)?;
assert!(
serialized.contains("custom-prop"),
"Expected serialized schema to include custom property key, but it was dropped. Serialized: {serialized}. Parsed schema: {schema:?}"
);
Ok(())
}
```

output:
```
Expected serialized schema to include custom property key, but it was dropped. Serialized: "long". Parsed schema: Long
```

Iceberg encodes types like timestamptz to Avro as a primitive long with additional properties (e.g. `{"type":"long","logicalType":"timestamp-micros","adjust-to-utc":true`} or similar depending on precision).

Since apache-avro cannot represent custom properties on primitive types, the `adjust-to-utc` flag is dropped during parsing and the original schema semantics cannot be preserved.

Start with Schema::parse, Schema::custom_attributes(), and serde_json serialization, using the provided test1 reproducer as the first check. Trace how object-form primitive schemas are represented and emitted. Done means custom properties, including adjust-to-utc, remain inspectable and are preserved during schema round-tripping.

Written by the indexing model from the issue text.