Fix TLS (`client_encryption_options`) configuration for CQL

[mpenick]

What this PR does:

CQL TLS configuration is now loaded from a yaml file specified using the system property stargate.cql.config_path. This very similar to cassandra.yaml, but it only contains the options that are specific to the CQL transport layer.

This also include a couple bug fixes:

• Removes usage of ClientWarn, this is handled by the persistence layer.
• Remove usage of DatabaseDescriptor which is not properly configured.

Substantial portions of the following files have been copied from C* 4.0.3:

• Config.java
• EncryptionOptions.java
• YamlConfigurationLoader.java
• SSLFactory.java

Which issue(s) this PR fixes: Fixes #1001

Checklist

• [x] Changes manually tested
• [x] Automated Tests added/updated
• [x] Documentation added/updated
• [x] Add an example cql.yaml file to resources
• [x] Add tests for client certificates
• [x] Fix licences on .java files (either Stargate or Apache Cassandra)

[tatu-at-datastax]

One thing that may or may not be relevant: instead of using SnakeYAML directly (btw do we not need a dependency from pom.xml to it? Or is it assumed we get it transitively from Cassandra), there's also Jackson YAML module:

https://github.com/FasterXML/jackson-dataformats-text/tree/2.14/yaml

which let's you use Jackson API to bind to POJOs, or Maps or whatever using YAML-backed ObjectMapper. That could simplify things a bit. It does use SnakeYAML for actual decoding fwtw.

[mpenick]

One thing that may or may not be relevant: instead of using SnakeYAML directly (btw do we not need a dependency from pom.xml to it? Or is it assumed we get it transitively from Cassandra), there's also Jackson YAML module:

https://github.com/FasterXML/jackson-dataformats-text/tree/2.14/yaml

which let's you use Jackson API to bind to POJOs, or Maps or whatever using YAML-backed ObjectMapper. That could simplify things a bit. It does use SnakeYAML for actual decoding fwtw.

Updated in this commit: https://github.com/stargate/stargate/pull/1992/commits/de500843beea07c377eaa381a46a6af01279d004