There’s a simple way to return a list of field configurations and field configuration schemes in Jira DC/Jira Server.  However, in order to find that information you have to know that Jira once referred to these as field layouts. 

Using the FieldLayoutManager class, this script returns a list of field layouts:

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.fields.layout.field.FieldLayoutManager

def layoutManager = ComponentAccessor.getFieldLayoutManager()
def fieldLayouts = layoutManager.getEditableFieldLayouts()
def sb = []

fieldLayouts.each{ fieldLayout ->

    sb.add("<br> ${fieldLayout.name}")

}

return sb

This script returns the field layout schemes, with a simple change of the method:

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.fields.layout.field.FieldLayoutManager

def layoutManager = ComponentAccessor.getFieldLayoutManager()
def layoutSchemes = layoutManager.getFieldLayoutSchemes()
def sb = []

layoutSchemes.each{ layoutScheme ->

    sb.add("<br> ${layoutScheme.name}")

}

return sb