New Config API: Re-evaluate public API & configuration options

[felixschlegel]

• Currently, we expose more or less all librdkafka configurations — do we want to narrow it down to the most basic configurations and expand this on user request / provide an unsafe API that allows pro-users to set arbitrary key-value options?
• librdkafka exposes configuration options regarding OAuth and SSL keys etc. — does our package even intend to expose that as well?

[felixschlegel]

Here is a list of the options we are currently not supporting but might want to support in the future:

(See librdkafka/CONFIGURATION.md for reference)

EDIT: since #88

• [ ] topic.metadata.refresh.fast.cnt
• [ ] connections.max.idle.ms
• [ ] api.version.request
• [ ] api.version.request.timeout.ms
• [ ] ssl.cipher.suites
• [ ] ssl.curves.list
• [ ] ssl.sigalgs.list
• [x] ssl.key.pem
• [x] ssl_key
• [x] ssl.certificate.pem
• [x] ssl_certificate
• [x] ssl.ca.pem
• [x] ssl_ca
• [ ] ssl.ca.certificate.stores
• [ ] ssl.providers
• [ ] ssl.engine.location
• [ ] ssl.engine.id
• [ ] ssl_engine_callback_data
• [x] enable.ssl.certificate.verification
• [ ] ssl.endpoint.identification.algorithm
• [ ] ssl.certificate.verify_cb
• [x] sasl.mechanisms
• [x] sasl.kerberos.service.name
• [x] sasl.kerberos.principal
• [x] sasl.kerberos.kinit.cmd
• [x] sasl.kerberos.keytab
• [x] sasl.kerberos.min.time.before.relogin
• [x] sasl.oauthbearer.config
• [x] sasl.oauthbearer.method
• [x] sasl.oauthbearer.client.id
• [x] sasl.oauthbearer.client.secret
• [x] sasl.oauthbearer.scope
• [x] sasl.oauthbearer.extensions
• [x] sasl.oauthbearer.token.endpoint.url
• [ ] group.instance.id
• [ ] partition.assignment.strategy
• [ ] coordinator.query.interval.ms
• [ ] queued.min.messages
• [ ] queued.max.messages.kbytes
• [ ] fetch.message.max.bytes
• [ ] max.partition.fetch.bytes
• [ ] fetch.min.bytes
• [ ] fetch.max.bytes
• [ ] fetch.error.backoff.ms
• [ ] check.crcs
• [ ] client.rack
• [ ] retry.backoff.ms
• [ ] compression.codec
• [ ] compression.type
• [ ] batch.num.messages
• [ ] batch.size
• [ ] sticky.partitioning.linger.ms