Cherry-pick of PR #18854 to 7.7 branch. Original message:

• Bug

What does this PR do?

Add support for configuring the string casing in the index/pipeline/key/topic 'Selector'.

Why is it important?

Elasticsearch pipeline and index names are required to be lower case only. When used with fields from events this was not always guaranteed, leading us to enforce lower case always (#16081. #6342).
As the code is reused for Kafka topic selection, this unfortunately did lead to a Regression as some users expect strings to allow mixed case (#18640).
With this PR Elasticsearch related resources (e.g. index or pipeline names) are set to lowercase only, while not touching the strings in other outputs.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
  - [ ] I have made corresponding changes to the documentation
  - [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

1. configure the Elasticsearch output while configuring an index name upper case characters => Check the index has only lower case characters in Elasticsearch
2. configure the Kafka output with topic name having upper case characters => Check the name is not modified in Kafka.

Related issues

• Closes [Winlogbeat] Output/Kafka Can’t use topics with uppercase letters anymore #18640