build: Introduce Tool for OTel Engine Generation Instead of Generate File

[blewis12]

Brief description of Pull Request

The original purpose of this branch was to add a step before generating the collector that will remove all generated files so that we're generating "from scratch". This is because there have been a couple times that modifications to the alloy and alloy_engine go.mod files have broken the go.mod file in the collector/ directory. Annoyingly, this collector/go.mod file has to be valid in order for the OCB generation to run to fix things - I think this is because running go commands first checks if the relevant go.mod file is valid, so we run into a chicken vs egg situation

The way to avoid this is to ensure that code generation runs with a separate go.mod file, outside of the collector directory. This way it doesn't matter if the collector go.mod file is valid or not in order to re-generate itself. This PR therefore moves the generation code into tools/generate-otel-engine-collector - which like tools/generate-module-dependencies is implemented as a CLI tool. The steps are the exact same as before but with two differences:

1. We now have a --from-scratch option which is enabled by default, which will remove generated files before re-generating them to avoid invalid states
2. We call the OCB tool via code instead of via go generate

TODO:

• Do we actually need --from-scratch or is having a separate go.mod file managing the tooling enough?
• Could we use magefiles? having a CLI feels kinda hard to read