Allow overriding built-in type names

As mentioned by @KitFieldhouse in https://github.com/apache/avro-rs/issues/476#issuecomment-4009905613 this test should pass:

```rust
#[test]
fn avro_rs_508_allow_overriding_type_names() -> TestResult {
let schemas = Schema::parse_list([
r#"{
"type": "record",
"name": "fixed",
"fields": [{
"name": "foo",
"type": "string"
}]
}"#,
r#"{
"type": "record",
"name": "Spam",
"fields": [{
"name": "bar",
"type": "fixed"
}]
}"#
])?;

Ok(())
}
```

Start with the provided avro_rs_508_allow_overriding_type_names test and the Schema::parse_list entry point. Trace schema name resolution for the two supplied records, then verify that the test passes with a record named "fixed" referenced as a field type.

Written by the indexing model from the issue text.