(aws-efs): Support for attaching multile file systems to lambda function

Support configuring multiple file systems as available in CFN: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-lambda-function.html#cfn-lambda-function-filesystemconfigs

Currently we can only specify one file system:

https://github.com/aws/aws-cdk/blob/8ed088a4cc5388b85776c3486cb657462836d923/packages/%40aws-cdk/aws-lambda/lib/function.ts#L280

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*Originaly reported as*:

(aws-efs): unable to mount more than one efs with lambda function

I can mount multiple efs volumes manually for a lambda but using the ts-cdk library I cannot mount more than a single efs volume.

Read the CloudFormation FileSystemConfigs documentation first, then inspect packages/@aws-cdk/aws-lambda/lib/function.ts at the linked definition and its surrounding file-system configuration. Done means the TypeScript CDK API supports configuring more than one file system for a Lambda function and represents the corresponding multiple configurations.

Written by the indexing model from the issue text.