Add X509::Certificate#tbs_bytes

[segiddins]

Ref #519

This makes verifying embedded certificate transparency signatures significantly easier, as otherwise the alternative was manipulating the ASN1 sequence, as in sigstore/sigstore-ruby@656d992

[rhenium]

Thank you for working on this!

[rhenium]

GitHub Actions is failing with LibreSSL 3.5.3 because the libressl? helper method wants 3 arguments (as LibreSSL doesn't employ semver). It was added in 3.5.0, according to the ChangeLog.

16) Error: test_tbs_precert_bytes(OpenSSL::TestX509Certificate): NoMethodError: undefined method `>=' for nil:NilClass
/home/runner/work/openssl/openssl/test/openssl/utils.rb:120:in `libressl?'
/home/runner/work/openssl/openssl/test/openssl/test_x509cert.rb:326:in `test_tbs_precert_bytes'
     323:   end
     324: 
     325:   def test_tbs_precert_bytes
  => 326:     pend "LibreSSL < 3.5 does not have i2d_re_X509_tbs" if libressl? && !libressl?(3, 5)
     327: 
     328:     cert = issue_cert(@ca, @rsa2048, 1, [], nil, nil)
     329:     seq = OpenSSL::ASN1.decode(cert.tbs_bytes)

Let me fix this locally.