Goal

When moving to the new lambda provider, we want to avoid all imports which rely on the logic of the old provider, to make seamless switching possible

Main changes

• Introduced a new environment variable, MAIN_DOCKER_NETWORK in order to replace the usage of LAMBDA_DOCKER_NETWORK for other services as well (as it is a misnomer, and to avoid tight coupling of lambda with other services). For now, until a major/minor version release, this variable falls back to LAMBDA_DOCKER_NETWORK to simulate current behavior. /cc @thrau

Splitting other services from lambda modules

• SNS: Move process_sns_notification_to_lambda to sns module (it belongs there thematically, since SNS is responsible for the payload) and invoke using boto client /cc @bentsku
• DynamoDB: Use botocall to list event source mappings instead of accessing the data directly /cc @giograno
• ApiGateway: Move invocation logic (payload etc.) to apigateway module, use boto client to invoke. Also some changes regarding base64 encoded data (which should be present in the json as string, otherwise json.dumps will not work), and removal of processing of everything not a boto response (flaskresponses, lambda invocation results etc). Also moved a test (for the apigw event payload) from lambda unit tests to apigatway unit tests (also belongs there thematically) /cc @calvernaz
• snapshots: Move account_id fixture to fixtures.py, since it is universally applicable /cc @dominikschubert
• docker_utils: Move get_host_path_for_path_in_docker into docker utils, since other services need this as well. Also change some comments insinuating strictly lambda usage /cc @thrau
• stepfunctions: Remove direct module access to lambda api internals for asserting stepfunctions invoking lambdas. The existing assertions already assert the results, so the lambda invocation in the middle is asserted with it as well. /cc @dominikschubert