fix: Update the Schema GetHashCode for Dictionary lookups

[seanamorosoamtote]

What

The GetHashCode for the Schema data contract was updated in June with additional properties to accomodate the DataContract work. However, the CachedSchemaRegistryClient is not able to retrieve items stored in its dictionary cache because of the References addition and how List<>'s are handled.

We saw severe slow down (around 15 seconds at times) registering a schema with 13 references (and their recursive references) because it was trying to query schemas that it already had in the Dictionary. When the changes proposed were made, the schemas that had been already retrieved were able to be pulled from the cache again (down to 1-2 seconds for large schemas, under a second for most).

Feel free to update as you wish but the code you need to ensure works is at src/Confluent.SchemaRegistry/CachedSchemaRegistryClient.cs

 if (!idBySchema.TryGetValue(schema, out int schemaId))
  {
      CleanCacheIfFull();

      schemaId = await restService.RegisterSchemaAsync(subject, schema, normalize)
          .ConfigureAwait(continueOnCapturedContext: false);
      idBySchema[schema] = schemaId;
  }

The if statement above would never get a schemaId from the idBySchema lookup.

Checklist

• Contains customer facing changes? Including API/behavior changes
• Did you add sufficient unit test and/or integration test coverage for this PR?
  • If not, please explain why it is not required

I don't see any unit tests surrounding this code anywhere or I would.

References

JIRA:

Test & Review

Serialize an object and debug the CachedSchemaRegistryClient to ensure that when a schema has been retrieved and/or registered it is able to be pulled from the dictionary.

Open questions / Follow-ups

[rayokota]

/sem-approve

[rayokota]

Thanks @seanamorosoamtote , LGTM