Add string truncate processor to the family of mutate string processor

[kkondaka]

Is your feature request related to a problem? Please describe.
The string truncate processor should truncate the string to the user configured length.

Describe the solution you'd like
Solution would be to support truncate string processor with the following config

  processor:                                                                                                                                            
    - truncate:                                                                                                                                
        entries:                                                                                                                                        
          - source: "message" 
            start_at: 2                                                                                                                      
            length: 5                                                                                                                                  
            truncate_when: <condition>  

Describe alternatives you've considered (Optional)
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.