Add or/and/not in processors

[monicasarbu]

This meta issue has the scope of documenting how to add support for and, or and not support in the condition associated to the processors.

Let's start with a bit of overview of the processors configuration option.

The general definition of processors is the following:

processors:
   - <processor>:
       when:
         <condition>
       <parameters>

where:
<processor> can be one of: drop_event, drop_fields, include_fields.
<condition> contains the definition of the condition that needs to be fulfilled in order for the processor to be executed. If no condition is present, the processor is always executed.
and <parameters> is the list of parameters that are executed by the processor when the condition is fulfilled.

The signature for or, and and not is:

processors:
  - <processor>:
     when:
       or/and/not:
         - <condition>
         - <condition>
         - <condition>

As an example, the condition a1 or a2 and a3 translates to:

processors:
   - <processor>:
     when:
        or:
          - a1
          - and:
             - a2
             - a3

[monicasarbu]

Duplicates #1447