Use plural names for plural fields

[dlvenable]

Is your feature request related to a problem? Please describe.

Data Prepper's configuration takes a list of processors and a list of sinks. However, these are both named processor and sink respectively. If something is plural, the configuration option should be plural.

Describe the solution you'd like

Add new aliases for processors and sinks in a 2.x version of Data Prepper.

Remove the singular names in a future major version (preferably 3.0).

Describe alternatives you've considered (Optional)

N/A

Additional context

We have already done this for route and routes. See #2520.