Make the runner more flexible for easier SQRL integration

[ferenc-csaky]

After integrating this into the sqrl infra, I ended up fine-tuning some things and adding a bit more logic here to make things more flexible.

Main changes

• Reworked the main class to 2 different implementations, CliRunner that is intended for standalone use, and JavaRunner that is intended for the SQRL test env. (Now that I wrote this, maybe SqrlRunner is a better name.)
• The common logic is put into a BaseRunner abstract class.
• Updated EnvVarResolver to a non-static utility, cause we need to inject environment variables on the SQRL side, and to keep that with the JSON deserializer, this seemed more appropriate.
• Introduced default value handling into EnvVarResolver, that can be given via ${KEY:defaultValue}.
• Added a static factory method to SqlExecutor that puts UDFs into a user class loader, and adapt the Flink conf accordingly as well, cause the ADD JAR method does not work on the SQRL test env.
• Covered newly added logic with unit tests.