chore(scripts): simplify mock.gen.sh to use go generate commands

[qdm12]

Why this should be merged

⚠️ Blocked by ava-labs/avalanchego#3628

💁 Sibling PR for subnet-evm so both repositories would look similar in this aspect

• Localized mockgen commands in the package where they are needed
• Generate mocks from your IDE directly
• Platform independent way of generating mocks
• Check automatically mocks are kept up to date
• Check automatically mocks without a source controlled generation command
• Remove license headers for generated mocks

How this works

• New mocks_generate_test.go file per package where mocks need to be generated, containing only //go:generate commands for mock generation. Each command is relative to the current package directory
• Use //go:generate go run go.uber.org/mock/mockgen@v0.4 to avoid requiring to pre-install mockgen
• no shell script, just go command needed with go generate -run "mockgen" ./...
  • Remove now unneeded scripts/mocks.gen.sh and scripts/mocks.mockgen.txt
• Add step to CI test job to check:
  • mocks are up to date, by re-generating all of them and running a git diff
  • mocks each have a source controlled //go:generate generation command, by removing all of them before re-generating them
• Documentation added to .github/CONTRIBUTING.md

How this was tested

• Locally with go generate -run mockgen ./...
• CI failure: https://github.com/ava-labs/coreth/actions/runs/12549009943/job/34989287671?pr=721

Need to be documented?

Yes in .github/CONTRIBUTING.md

Need to update RELEASES.md?

Not really

[darioush]

IMO the license header is not needed, otherwise it's a great simplification.

[darioush]

Let's add the CI check to this PR as well and close 720.