fix: add 'prefix' to config schema (fixes #562)

- Although not documented in the readme, there are code paths that allow prefix to be overridden via config. I'm unsure whether this was a feature that was only partially implemented, or maybe an existing feature that was broken at some point, so to be on the safe side, I'll add it to the config schema, but leave it undocumented for now.

lib/serverless-sns-sqs-lambda.ts

@@ -170,6 +170,7 @@ export default class ServerlessSnsSqsLambda {
       properties: {
         name: { type: "string" },
         topicArn: { $ref: "#/definitions/awsArn" },
+        prefix: { type: "string" },
         omitPhysicalId: { type: "boolean" },
         batchSize: { type: "number", minimum: 1, maximum: 10000 },
         maximumBatchingWindowInSeconds: {