refactor: Import upstream in as dependency and rework LocalStack extensions into modules

[gregfurman]

Changes

• Uses the replace directive to import in the upstream aws/aws-lambda-runtime-interface as a Golang package/dependency as opposed to relying on the fork relationship.
  • In go.mod: replace go.amzn.com => github.com/aws/aws-lambda-runtime-interface-emulator v0.0.0-20250423173140-3a0772eae98d (v0.0.0-20250423173140-3a0772eae98d is a SHA pointing to the latest tagged version).
• Reworked LocalStack extension code into modules instead of all being in the main package.
• All extension code now resides in internal/ so as to restrict it being imported outside of this repo.

TODO

• Simplify CustomInteropServer implementation.
• Improve usage of dependency injection throughout codebase.
• Create structs/models for many of these modules.
• Add tests.
• Update documentation to remove references to the upstream where appropriate.