[lambda] specify multiple folders as a single lambda asset

Hello,
Great framework, keep up the good work.

Right now in order to define a lambda function you need to set where the code resides, e.g. `code: lambda.Code.asset('lambda')`. Sometimes you would like to define multiple folders, for example the actual code and `node_modules`, although packing can be done as a preprocess step before actually running `cdk deploy`, it would be cleaner to do it as part of the lambda configuration.

Start by reading the Lambda asset configuration around `lambda.Code.asset('lambda')` and the packaging behavior used before `cdk deploy`. Determine how multiple folders, including the function code and `node_modules`, should be represented as one asset. Done means the Lambda configuration can package those folders together without a separate preprocessing step.

Written by the indexing model from the issue text.