Consolidate feature flags into a single registry

[mzihlmann]

Fixes #852

The two flags in #852 declared their default at each read site, so the same flag could be read with conflicting defaults, which is what caused the panic and the self-contradictory deprecation warning. Rather than patch the individual call sites, this moves every FF_KANIKO_* flag into a single registry in pkg/config. The environment is read once at startup into a FeatureFlags struct and call sites just read config.FF.X, so a flag's default has exactly one source of truth and the whole class of drift becomes impossible. Startup now logs which flags are active, warns when a default-on flag is explicitly disabled, and warns when an unknown FF_KANIKO_* variable is set; a completeness check guarantees every struct field is wired to a flag so a new flag cannot be left uninitialised. The assertion helper moved into its own pkg/assert because the registry guard needs it and pkg/util already depends on pkg/config, so it could not live in util without an import cycle.

Supersedes #853, which patched the two call sites directly.

[codecov]

Codecov Report

❌ Patch coverage is 88.62275% with 19 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/config/featureflags.go	85.48%	4 Missing and 5 partials ⚠️
pkg/executor/build.go	88.88%	3 Missing and 2 partials ⚠️
cmd/executor/cmd/root.go	66.66%	0 Missing and 1 partial ⚠️
pkg/cache/cache.go	50.00%	1 Missing ⚠️
pkg/snapshot/layered_map.go	75.00%	1 Missing ⚠️
pkg/util/transport_util.go	0.00%	0 Missing and 1 partial ⚠️
pkg/warmer/warm.go	66.66%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!