Append values to lists in an event

[oeyh]

Is your feature request related to a problem? Please describe.
Append is a common operation for lists. As a Data Prepper user, I would like to be able to append values to a list in the event.

Describe the solution you'd like
add_entries processor can be enhanced to achieve this by adding an append_if_key_exists option:

    - add_entries:
        entries:
          - key: "fruits"
            value: "banana"
            append_if_key_exists: true

Example1:
Input:

{"fruits": "apple"}

output:

{"fruits": ["apple", "banana"]}

Example2:
Input:

{"fruits": ["apple", "watermelon"]}

output:

{"fruits": ["apple", "watermelon", "banana"]}

Describe alternatives you've considered (Optional)
A separate list_append processor.

Additional context
#3967

[dlvenable]

@oeyh , What is the default behavior now? I tend to think that this behavior should be the default unless overwrite_if_key_exists is set to true.

[oeyh]

@oeyh , What is the default behavior now? I tend to think that this behavior should be the default unless overwrite_if_key_exists is set to true.

@dlvenable Currently, the value will not be added if the key exists and overwrite_if_key_exists is not set.