feat: 👷🔒 Security scans workflow

[evaline-ju]

Summary

• Adds permissions and dependency pins for existing CI workflow for linting, unit tests
• Adds security scan workflow, borrowed liberally from https://github.com/kagenti/kagenti/blob/main/.github/workflows/security-scans.yaml. Note gitleaks as commented in main kagenti would require a license
• Start to address security scan issues - the readOnlyRootFilesystem for the ext-proc deployment was not quick to fix, so this is added to a .trivyignore ignore file for now
• proto-build.sh script had to be updated to work with the "latest" envoy_data_plane library used to help build protos for this project, as it was not previously pinned. Team members say this was working with commit "bb0c0f1e912faa641cb5d9ccf1e2ef002945467f" (one commit after tag: 2.0.0b1 previously). The "latest" tag for which this is working is now incorporated as a "checkout" in the script

Related issue(s)

Addresses part of #55

[github-advanced-security]

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