fix(ocf): default missing avro.codec to null

[pinanek]

According to the Apache Avro specification, the avro.codec metadata entry is optional. When it is absent, readers should treat the codec as null.

The current OCF metadata implementation requires avro.codec to be present:

#[derive(serde_derive::Deserialize, serde_derive::Serialize)]
struct Metadata<S, M> {
	#[serde(rename = "avro.schema")]
	schema: S,

	#[serde(rename = "avro.codec")]
	codec: CompressionCodec,

	#[serde(flatten)]
	user_metadata: M,
}

As a result, valid Avro Object Container Files that omit avro.codec fail to deserialize with a missing-field error.

This PR changes CompressionCodec to default to CompressionCodec::Null and marks the codec field with #[serde(default)].

[Ten0]

I'm not fond of the new public Default impl, but agree this should be fixed in the Deserialize impl.
Can you propose a fix that doesn't add this Default impl? (e.g. does Serde allow specifying the default? Otherwise we could just make the field an option since that struct is private anyway)

[Ten0]

Oh wait that is private anyway. OK then my only question is can't we derive default instead of implementing it manually?

[pinanek]

Ah, I missed that. Yeah, deriving Default is cleaner. I’ll update it.

[Ten0]

released as 2.1.2