🐛 fall back to system trust store when no custom TLS CA is set (#331)

* 🐛 fall back to system trust store when no custom TLS CA is set

Closes #327.

Until now, configuring `crowdsecLapiScheme=https` forced the operator
to either provide `crowdsecLapiTLSCertificateAuthority` (a custom CA)
or set `crowdsecLapiTLSInsecureVerify=true` — there was no way to rely
on the host's system trust store, which is the expected setup when the
LAPI sits behind a reverse proxy with a publicly trusted (e.g. Let's
Encrypt) certificate.

Two contributing bugs:
  - `validateParamsTLS` rejected an empty CA up-front.
  - `getTLSConfig` always allocated an empty `tls.Config.RootCAs`,
    which silently disabled the standard library's fall-back to
    `x509.SystemCertPool()`.

Fix: drop the validation error for the empty-CA case and only allocate
`RootCAs` when a custom CA is actually provided. Same change applies
symmetrically to the AppSec path since the helper is shared.

Add unit tests covering the four meaningful states (HTTP, HTTPS with
system CA, HTTPS with custom CA, HTTPS with insecure verify) plus the
malformed-PEM rejection. README updated to document the system trust
store as an explicit option for both LAPI and AppSec HTTPS.

* 📝 fix gofmt alignment in TLS test struct

* ✅ update existing test: https without CA is now accepted

* ♻️ tests: hoist shared validPEM to package level, rename cfgGarbage

Address review on #331:
- the self-signed validPEM block was duplicated in two test funcs; declare it
  once at package level and drop both local copies.
- rename cfgGarbage -> cfgInvalidCA (and its test case) for a descriptive name.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* ✅ e2e mock: add tls-system-ca scenario (HTTPS LAPI via system trust store)

CI regression coverage for this PR: with no custom CA configured, the bouncer
must fall back to the OS/system trust store for an HTTPS LAPI.

In the binary suite the "system trust store" is whatever Go's
x509.SystemCertPool() reads, which honours SSL_CERT_FILE on the Traefik process.
The scenario mints a throwaway CA, serves the mock LAPI over HTTPS with a cert
signed by it, and runs the stack twice:
  - positive: SSL_CERT_FILE = our CA       -> LAPI trusted    -> 200
  - negative: SSL_CERT_FILE = empty bundle  -> not trusted     -> 403 (fail-closed)
The negative run proves the patch still VERIFIES (not an insecure skip).

- mocklapi: optional --lapi-tls-cert/--lapi-tls-key to serve the LAPI over TLS.
- common.sh: opt-in LAPI_TLS_CERT/KEY (HTTPS mock) and TRAEFIK_SSL_CERT_FILE
  (inject SSL_CERT_FILE into Traefik); both default-empty, other scenarios
  unaffected.
- adds openssl as a scenario-only dependency.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

tests/e2e/mock/lib/common.sh

@@ -190,16 +190,29 @@ start_stack() {
     -e "s|@@SCENARIO_DIR@@|${scenario_dir}|g" \
     "$scenario_dir/dynamic.yml" > "$WORKDIR/dynamic.yml"
 
+  # Opt-in HTTPS LAPI: a scenario exports LAPI_TLS_CERT/LAPI_TLS_KEY to serve the
+  # LAPI over TLS (used by tls-system-ca). Default empty -> plaintext as before.
+  local mock_tls_args=() lapi_scheme=http lapi_curl=()
+  if [[ -n "${LAPI_TLS_CERT:-}" && -n "${LAPI_TLS_KEY:-}" ]]; then
+    mock_tls_args=(--lapi-tls-cert "$LAPI_TLS_CERT" --lapi-tls-key "$LAPI_TLS_KEY")
+    lapi_scheme=https
+    lapi_curl=(-k) # the readiness probe ignores trust; the bouncer's trust is what we test
+  fi
+
   "$mock_bin" \
     --lapi-addr "127.0.0.1:${LAPI_PORT}" \
     --backend-addr "127.0.0.1:${BACKEND_PORT}" \
-    --appsec-addr "127.0.0.1:${APPSEC_PORT}" >"$WORKDIR/mock.log" 2>&1 &
+    --appsec-addr "127.0.0.1:${APPSEC_PORT}" \
+    "${mock_tls_args[@]}" >"$WORKDIR/mock.log" 2>&1 &
   MOCK_PID=$!
 
-  ( cd "$WORKDIR" && exec "$traefik_bin" --configfile=traefik.yml ) >"$WORKDIR/traefik.log" 2>&1 &
+  # Opt-in trust store for the Traefik process: a scenario exports
+  # TRAEFIK_SSL_CERT_FILE to point Go's x509.SystemCertPool() at a specific CA
+  # bundle. Empty -> Go's default system store (unchanged behaviour).
+  ( cd "$WORKDIR" && SSL_CERT_FILE="${TRAEFIK_SSL_CERT_FILE:-}" exec "$traefik_bin" --configfile=traefik.yml ) >"$WORKDIR/traefik.log" 2>&1 &
   TRAEFIK_PID=$!
 
-  wait_for_status "http://127.0.0.1:${LAPI_PORT}/health" 200 30
+  wait_for_status "${lapi_scheme}://127.0.0.1:${LAPI_PORT}/health" 200 30 "${lapi_curl[@]}"
   # AppSec stand-in: a bare GET carries no "rpc2" URI, so it answers 200 (allow).
   wait_for_status "http://127.0.0.1:${APPSEC_PORT}/" 200 30
   # /ping is served by Traefik itself once it is up (plugin compilation included).