Incorporate DHS' function builder into Lambda deployment

[konklone]

The tools at https://github.com/dhs-ncats/gatherer are very helpful, and use Docker to build Lambda functions rather than the unscripted and ad hoc process this repo uses now.

Instead, we should support something like:

./lambda/deploy pshtt --build

Which would trigger a full rebuild of the Lambda function. Alternatively, we could do a ./lambda/build script as a separate thing from ./lambda/deploy. (I like this better.)

[konklone]

Note that #252 did this from a documentation perspective. This ticket could still track greater integration.

cc @jsf9k @buckley-w-david