To ensure that assets are available for the toolchain to deploy after the CDK app
exists, the CLI will, by default, request that the app will stage the assets under
the `.cdk.assets` directory (relative to working directory).

The CDK will then *copy* all assets from their source locations to this staging
directory and will refer to the staging location as the asset path. Assets will
be stored using their content fingerprint (md5 hash) so they will never be copied
twice unless they change.

Docker build context directories will also be staged.

Staging is disabled by default and in cdk-integ.

Added .cdk.staging to all .gitignore files in cdk init templates.

Fixes #1716

…2027)

This Code type is helpful when there is no local file/directory to use Assets with,
and the Lambda is supposed to be deployed in a CodePipeline.

Until aws/jsii#442 is released
we must fail builds on compilation errors. This is a temporary
workaround for this issue (if there is something in stdout
we consider that a compilation error).