Add migrate process plugin for concatenating the value with a string

I don't only post on d.o. for others, but for myself when I forget how I did things :-)

An example of concatenating a file description value with the node title and a constant using include_source (after trying a whole bunch of process chains that did not work). There could be a better way, but I haven't found it.

Hope this help someone further.

source:
  constants:
    factsheet: 'Factsheet'

field_file:
  - plugin: sub_process
    include_source: true
    source_key: node_source
    source: field_some_file
    process:
      target_id: fid
      display: display
      description:
        - plugin: concat
          source:
            - 'node_source/title'
            - 'node_source/constants/factsheet'
          delimiter: ' '