Support default route option for Events that match no other route

[graytaylor0]

Is your feature request related to a problem? Please describe.
As a user of data prepper, I would like to be able to configure a default route for when my Events do not match any of my other routes. Currently, I would have to add an expression manually to handle this case.

Describe the solution you'd like
An option to use a custom route value to use as the default. Routes with a value of default or DEFAULT will be this default route

routes:
  - ROUTE_ONE: '/my_key == null`
  - DEFAULT_ROUTE: default
sink:
 - opensearch:
      routes:
         - ROUTE_ONE
  - opensearch:
       routes:
          - DEFAULT_ROUTE

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.

[dlvenable]

Could this make sense? Have a special pre-defined route named _default or just default

routes:
  - ROUTE_ONE: '/my_key == null`
sink:
 - opensearch:
      routes:
         - ROUTE_ONE
  - opensearch:
       routes:
          - _default

The routes list is a list of expressions. But, default isn't an expression, so it would not fit with the current definition of what these are.