[WIP] KAFKA-19112 Unifying LIST-Type Configuration Validation and Default Values

[m1a2st]

We add the three main changes in this PR

• Disallowing null values for most LIST-type configurations makes sense,
  since users cannot explicitly set a configuration to null in a
  properties file. Therefore, only configurations with a default value of
  null should be allowed to accept null.
• Disallowing duplicate values is reasonable, as there are currently no
  known configurations in Kafka that require specifying the same value
  multiple times. Allowing duplicates is both rare in practice and
  potentially confusing to users.
• Disallowing empty list, even though many configurations currently
  accept them. In practice, setting an empty list for several of these
  configurations can lead to server startup failures or unexpected
  behavior. Therefore, enforcing non-empty lists helps prevent
  misconfiguration and improves system robustness.
  These changes may introduce some backward incompatibility, but this
  trade-off is justified by the significant improvements in safety,
  consistency, and overall user experience.

Additionally, we introduce two minor adjustments:

• Reclassify some STRING-type configurations as LIST-type, particularly
  those using comma-separated values to represent multiple entries. This
  change reflects the actual semantics used in Kafka.
• Update the default values for some configurations to better align with
  other configs.
  These changes will not introduce any compatibility issues.

[m1a2st]

The HeaderFrom fields configuration should allow duplicate values. Therefore, its default value should be NO_DEFAULT_VALUE, and its validator should be new NonEmptyListValidator().

Please refer this testcase:

kafka/connect/transforms/src/test/java/org/apache/kafka/connect/transforms/HeaderFromTest.java

Line 240 in 18045c6

// two headers from the same field

[m1a2st]

The bootstrap.servers configuration requires further discussion. If we set its default value to NO_DEFAULT_VALUE and use the validator ConfigDef.ValidList.anyNonDuplicateValues(false, false), initialization will fail because the default value does not pass the validator. One possible solution is to update the default value to List.of("localhost:9092").