feat: provider construct and filemanager migration

[mmalenic]

Closes #170

Changes

• Implements a provider function which calls CustomResource during CloudFormation deployment in order to migrate the filemanager database.
  • This is implemented to allow migration during deployment using CloudFormation Create, Delete and Update events.
  • The CustomResource is updated whenever the handler function changes using the asset hash, so that the function gets called again when it changes.
  • If the migration fails, the CloudFormation stack fails to deploy.
  • This replaces the non-working CdkResourceInvoke, which used a SingletonLambda that only ran once.
  • Note, this is a general construct in the components directory, so it could also be used by other services.

Checksums

• The filemanager database now tracks the SHA256 checksum if it is available (I mostly included this as a real-world test for the migration provider, as it changes the table).

Misc

• The filemanager compilation behaves a lot more nicely now.
  • Code is only compiled once, and there should be no more concurrency bugs because tests synth once. Related to test: avoid synthesizing the CDK stack in tests multiple times #190.
  • Only one postgres container is started by first checking if one is already running.
• Removed unused code, structs and functions. For example, env.rs is removed, because sqlx already sources the DATABASE_URL automatically.