Allow requestCert option to be set

[toddself]

Summary

Allow new SslConfig to accept the requestCert boolean option to be
passed into the constructor.

Checklist

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• This was checked for cross-browser compatibility, including a check against IE11
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Dev Docs

This allows us to require an HTTP server created by Kibana to force a
client to provide a certificate for authorization to the server
instance. This enables PKI-based mutual TLS for client/server
interactions

Creating a configuration as such:

const sslConfig = new SslConfig({
  requestCert: true,
  ca: [myCA],
  key: privateKey,
  cert: myCert
})

Would require a client to provide the same cert and key (and ca if
it's a self-signed certificate) to the https request in order to the
server to respond:

const agent = new https.Agent({
  key: privateKey,
  cert: myCert
  ca: myCa
})
https.request('https://kibana.local:3000/', { agent }).end()
// or
Wreck.defaults({
  agent: {
    https: agent
  }
})
await wreck.get('https://kibana.local:3000')

Without the same certificate, key and ca, the Kibana server will reject
the request as being unauthorized

[elasticmachine]

Pinging @elastic/kibana-platform

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[eliperelman]

LGTM.

[azasypkin]

@toddself Hmm, IIRC you either use schema.maybe(schema.boolean()) // requestCert?: boolean or just schema.boolean({ defaultValue: false }) // requestCert: boolean as having default value implies "optionality". Or I'm missing something?