Add variable to disable instrumentation

[clementdenis]

I want to be able to enable or disable instrumentation for my lambdas selectively based on which environment I deploy to.

It's currently inconvenient to do so using this module, as I would need to have a regular lambda resource when deploying without instrumentation, but would need to use the module when enabling. That's definitely possible using conditional activation with count attributes, but:

• It requires duplicated configuration for both regular and wrapped lambda
• enabling / disabling instrumentation would recreate the lambda (unless doing a manual move operation)

This PR adds a single disable_instrumentation variable to disable all customizations performed by the module, acting as a pass-through wrapper for the aws_lambda_function resource.