Make EventSourceMapping.starting_position optional.
Make EventSourceMapping.starting_position optional.

Amazon recently made it possible to have SQS events trigger Lambdas.  See: https://aws.amazon.com/blogs/aws/aws-lambda-adds-amazon-simple-queue-service-to-supported-event-sources/ for more details. 

As part of this they specifically utilize CreateSourceEventMapping to describe the mapping between SQS and Lambda: https://docs.aws.amazon.com/lambda/latest/dg/with-sqs.html

*However*, as part of this, they changed the StartingPosition message property from required to optional.  You can see that it was previous required here: https://web.archive.org/web/20171222204154/https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-lambda-eventsourcemapping.html

```
StartingPosition
    The position in the stream where Lambda starts reading. For valid values, see CreateEventSourceMapping in the AWS Lambda Developer Guide.
    Required: Yes
    Type: String
    Update requires: Replacement
```

But now is optional in the current set of docs:

```
StartingPosition
    The position in the stream where Lambda starts reading. For valid values, see CreateEventSourceMapping in the AWS Lambda Developer Guide.
    Required: No
    Type: String
    Update requires: Replacement
```

Unfortunately, this makes is (afaict) impossible to use terraform to configure this EventSourceMapping.  If i leave out the starting_position property, terraform complains with:

```
Plan apply failed:
Error creating Lambda event source mapping: ValidationException: 1 validation error detected: Value '' at 'startingPosition' failed to satisfy constraint: Member must satisfy enum value set: [LATEST, AT_TIMESTAMP, TRIM_HORIZON]
```

However, if i supply any of those, AWS itself errors out with:

```
Plan apply failed: 
Error creating Lambda event source mapping: InvalidParameterValueException: StartingPosition is not valid for SQS event sources.
```

This PR attempts to rectify things by making this property optional to match current AWS expectations.
nbaztec/terraform-provider-aws,kjmkznr/terraform-provider-aws,nbaztec/terraform-provider-aws,terraform-providers/terraform-provider-aws,nbaztec/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,nbaztec/terraform-provider-aws