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: ' '

I was looking for a simple way of wrapping latitude longitude values for WKT format, and this worked well. Sure, I should probably have programmed a custom solution, but this works fine:

source:
[...]
  constants:
    wkt_prefix: 'POINT('
    wkt_separator: ' '
    wkt_suffix: ')'

process:
  title: name
  field_map_position:
    plugin: concat
    source:
      - 'constants/wkt_prefix'
      - lon
      - 'constants/wkt_separator'
      - lat
      - 'constants/wkt_suffix'