@confluentinc/schemaregistry performance optimisations: sync serialise function without fetching of new shema

[Dhruv-Garg79]

Instead of fetching schema in serialise and using async function for it, it's much better to keep that function sync and fetch schemas separately.

If that is not possible, having a separate sync serialise function with separate function for prefetching schemas will be great.

What we have right now:

serialize(topic: string, msg: any): Promise<Buffer>;

What I am proposing:

serialize(topic: string, msg: any): Buffer;

fetchSchemaFor(topic: string); // even if this method is not provided, we can simply use original method for prefetching