[@types/aws-lambda]: Add extractedFields optional field (#27670)

* [@types/aws-lambda]: Add extractedFields optional field

Added an undocumented optional field to a datatype which is sent zipped and base64 encoded. This field is only available if users select certain options when creating a subscription filter.

You can see the field used if you create a subscription filter to elasticsearch and look at the source for the builtin lambda. I couldn't find any documentation for this field except in the terms "fields extracted" [here](https://docs.aws.amazon.com/AmazonCloudWatch/latest/logs/FilterAndPatternSyntax.html) and found some amazon code referencing it [here](https://github.com/amazon-archives/cloudwatch-logs-subscription-consumer/blob/master/src/test/resources/aws-lambda-log-example.json#L14).

I nearly went insane tracking this down.

* Narrow the type of extracted fields to string:string

* Add test for extracted fields

types/aws-lambda/aws-lambda-tests.ts

@@ -479,6 +479,7 @@ str = cloudwatchLogsDecodedData.messageType;
 str = cloudwatchLogsDecodedData.logEvents[0].id;
 num = cloudwatchLogsDecodedData.logEvents[0].timestamp;
 str = cloudwatchLogsDecodedData.logEvents[0].message;
+str = cloudwatchLogsDecodedData.logEvents[0].extractedFields!["example"];
 
 /* ClientContext */
 clientContextClient = clientCtx.client;

types/aws-lambda/index.d.ts

@@ -371,6 +371,7 @@ export interface CloudWatchLogsLogEvent {
     id: string;
     timestamp: number;
     message: string;
+    extractedFields?: {[key: string]: string};
 }
 
 // Context