Update the mutate string processors to use the EventKey.

[dlvenable]

Description

Change the source and keys properties for mutate string processors to use EventKey such that they are parsed by Data Prepper core. Also, use the TestEventFactory in the tests to avoid use of JacksonEvent directly. Removes an unused class.

Tested against a processor chain that looks like:

  processor:
    - split_string:
        entries:
          - source: message
            delimiter: ','
    - uppercase_string:
        with_keys:
          - 'message/0'
          - 'message/1'
    - lowercase_string:
        with_keys:
          - 'message/2'
          - 'message/3'
    - trim_string:
        with_keys:
          - 'message/4'
          - 'message/5'
    - substitute_string:
        entries:
          - source: 'message/6'
            from: 'a{1}'
            to: '!'

Issues Resolved

Contributes toward #4646

Check List

• [x] New functionality includes testing.
• [x] New functionality has a documentation issue. Please link to it in this PR.
  • [x] New functionality has javadoc added
• [x] Commits are signed with a real name per the DCO

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