Make OpenSSL an optional crypto backend (closes #297)

OpenSSL was mandatory; it is now one of three interchangeable backends.
libjwt builds and passes its test suite with any non-empty subset of
{OpenSSL, GnuTLS, MbedTLS} (default WITH_OPENSSL=ON).

Core decoupling:
- Replace the OPENSSL_cleanse-based jwt_scrub_and_free (used in ~60 sites
  across the core and every backend) with a portable jwt_cleanse; guard the
  <openssl/crypto.h> include under HAVE_OPENSSL and include <limits.h>
  explicitly so non-OpenSSL builds need no OpenSSL header.

Native key2jwk:
- Split openssl/jwk-export.c into a crypto-free common jwk-export.c (the new
  jwk_export_t struct + key2jwk_params ops member, JWK assembly, base64url
  encoding, kid via the backend RNG, and the HMAC fallback) plus native
  per-backend extractors for OpenSSL, GnuTLS, and MbedTLS. Drops the
  cross-backend dependency on openssl_key2jwk.

RSA-OAEP (SHA-1):
- The GnuTLS -> OpenSSL fallback for plain RSA-OAEP (which GnuTLS/nettle
  cannot perform), is now gated on HAVE_OPENSSL; without OpenSSL it fails
  cleanly instead of referencing an undefined symbol.

Build:
- Add WITH_OPENSSL (default ON); move OpenSSL detection, sources, links, and
  HAVE_CRYPTO inside if(WITH_OPENSSL); stop linking OpenSSL into the tools;
  make the pkg-config Requires conditional; require GnuTLS >= 3.8.4 for an
  OpenSSL-less GnuTLS build (older GnuTLS has no native JWK/JWE path).

CI:
- Add a build-linux-combos matrix exercising all seven backend combinations
  (build + full test suite) in a debian:forky container.

Tests cover each backend's native key2jwk, with skips for genuinely
unsupported cases (MbedTLS+EdDSA/RSA-PSS, native GnuTLS+secp256k1); the CLI
golden-JWKS test skips on backends that cannot convert every key type.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Ben Collins <bcollins@libjwt.io>