mTLS support

[rymsha]

Provide a way to specify client certificate, similar to how curl --cert option does.

[rymsha]

Limitations:

• RSA, DSA not supported. Only PKCS #8 - this is what okhttp library supports
• curl --cert supports encrypted keys, okhttp does not. So, pem files cannot contain -----BEGIN ENCRYPTED PRIVATE KEY-----. Such files must be decrypted
• intermediate certificates not implemented. I have no idea how often they are in use and how to implement them best. It seems like even curl developers change their mind. But it is certainly possible to do, if/when needed.

[rymsha]

Test

• download badssl.com-client.pem https://badssl.com/download/
• openssl pkey -in badssl.com-client.pem -out badssl.com-client.decrypted.key (password is badssl.com) https://github.com/chromium/badssl.com/issues/369
• concatenate certificate and decrypted key into clientCertrificate

-----BEGIN PRIVATE KEY-----
MIIEvQIBADANBgkqhkiG9w0BAQEFAASCBKcwggSjAgEAAoIBAQDHN18R6x5Oz+u6
...
GTH3fhaM/pZZGdIC75x/69Y=
-----END PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
MIIEqDCCApCgAwIBAgIUK5Ns4y2CzosB/ZoFlaxjZqoBTIIwDQYJKoZIhvcNAQEL
...
g0Y2YBH5v0xmi8sYU7weOcwynkjZARpUltBUQ0pWCF5uJsEB8uE8PPDD3c4=
-----END CERTIFICATE-----

run

 var clientCertificate = io.newStream(/*clientCertificate string goes here*/)
  var result = http.request({
      url: 'https://client.badssl.com',
      clientCertificate: clientCertificate
  });
  log.info(JSON.stringify(result))

resutl.status should be 200