[SPARK-28695][SS] Use CaseInsensitiveMap in KafkaSourceProvider to make source param handling more robust

[gaborgsomogyi]

What changes were proposed in this pull request?

SPARK-28163 fixed a bug and during the analysis we've concluded it would be more robust to use CaseInsensitiveMap inside Kafka source. This case less lower/upper case problem would rise in the future.

Please note this PR doesn't intend to solve any kind of actual problem but finish the concept added in SPARK-28163 (in a fix PR I didn't want to add too invasive changes). In this PR I've changed Map[String, String] to CaseInsensitiveMap[String] to enforce the usage. These are the main use-cases:

• contains => CaseInsensitiveMap solves it
• get... => CaseInsensitiveMap solves it
• filter => keys must be converted to lowercase because there is no guarantee that the incoming map has such key set
• find => keys must be converted to lowercase because there is no guarantee that the incoming map has such key set
• passing parameters to Kafka consumer/producer => keys must be converted to lowercase because there is no guarantee that the incoming map has such key set

How was this patch tested?

Existing unit tests.

[SparkQA]

Test build #108969 has finished for PR 25418 at commit 2925640.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #108971 has finished for PR 25418 at commit d5ba9f1.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[gaborgsomogyi]

cc @HeartSaVioR @HyukjinKwon @cloud-fan this is a follow-up on #24967

[SparkQA]

Test build #109039 has finished for PR 25418 at commit c0b945c.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[cloud-fan]

thanks, merging to master!