Add escape character handling to key_value pipeline function (6.0)

[graylog-internal-actions-access]

Note: This is a backport of #21456 to 6.0.

Description

Adds a new use_escape_char parameter that turns on escape character handling for values in the key_value pipeline function.

• Added this feature as an optional parameter to avoid changing any existing usages of the key_value function.
• Only works on the outer splitter to allow values to contain escaped delimiter chars - ie, trying to use an escape char in the key doesn't work. Not sure that use case would make sense anyway but open to feedback.

Motivation and Context

closes Graylog2/graylog-plugin-enterprise#9552

How Has This Been Tested?

unit test

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.