Changing delimiter char for field/decorator in Scripting API

[janheise]

Description

This PR changes the delimiter char from . to # in the Scripting API for field/decorator usage. . is causing problems when querying for nested fields.

Although it's breaking change, I do not think it's going to have any impact on existing customers as it's only marginally mentioned in the docs.
https://go2docs.graylog.org/current/making_sense_of_your_log_data/simple_search_scripting_api.htm

This PR also adds a convenience method that is used in Enterprise.

Motivation and Context

How Has This Been Tested?

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 requested a documentation update.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.