Fix: insufficient test coverage in Snowflake

[misolt]

Increase test coverage in snowflake

• Make all classes with 10+ lines of logic (as counted by JaCoCo) meet the current target of 50% coverage.
• Overall, increase the average coverage for snowflake from 52 to 78%.

Make SnowflakeLogsConnector more testable

• Move bits of logic into helper functions that are easier to test.
• Use enum abstract methods instead of switches.
• Use fields to store constants with long definitions
• Minimize duplication in hard-to-cover parts of code.

Add tests for Snowflake connector logic

Add to the tests of metadata derivative connectors, logs and abstract connector. Add a util to avoid repeating the redundant throws IOException each test.

Other fixes

• Prevent using derivative metadata connectors with the assessment flag, since assessment doesn't support them.
• Move SnowflakeLogsConnectorProperties to a separate class.
• Remove ConnectorArguments#optionAsOptional.
• Move a comment to a more relevant context.